diff --git a/packages/plugin-ext/src/common/proxy-handler.ts b/packages/plugin-ext/src/common/proxy-handler.ts
index dcfe7d4a2e722..9a3f362ca7deb 100644
--- a/packages/plugin-ext/src/common/proxy-handler.ts
+++ b/packages/plugin-ext/src/common/proxy-handler.ts
@@ -25,11 +25,18 @@ export interface RpcHandlerOptions {
 }
 export interface ProxyHandlerOptions extends RpcHandlerOptions {
     channelProvider: () => Promise<Channel>,
+    proxySynchronizer: ProxySynchronizer,
 }
 
 export interface InvocationHandlerOptions extends RpcHandlerOptions {
     target: any
 }
+
+export interface ProxySynchronizer {
+    startProxyInitialization(id: string, init: Promise<void>): void
+    pendingProxyInitializations(): Promise<void>
+}
+
 /**
  * A proxy handler that will send any method invocation on the proxied object
  * as a rcp protocol message over a channel.
@@ -40,6 +47,7 @@ export class ClientProxyHandler<T extends object> implements ProxyHandler<T> {
 
     readonly id: string;
     private readonly channelProvider: () => Promise<Channel>;
+    private readonly proxySynchronizer: ProxySynchronizer;
     private readonly encoder: RpcMessageEncoder;
     private readonly decoder: RpcMessageDecoder;
 
@@ -50,6 +58,7 @@ export class ClientProxyHandler<T extends object> implements ProxyHandler<T> {
     private initializeRpc(): void {
         // we need to set the flag to true before waiting for the channel provider. Otherwise `get` might
         // get called again and we'll try to open a channel more than once
+        this.proxySynchronizer.startProxyInitialization(this.id, this.rpcDeferred.promise.then(() => { }));
         this.isRpcInitialized = true;
         const clientOptions: RpcProtocolOptions = { encoder: this.encoder, decoder: this.decoder, mode: 'clientOnly' };
         this.channelProvider().then(channel => {
@@ -69,7 +78,7 @@ export class ClientProxyHandler<T extends object> implements ProxyHandler<T> {
         const isNotify = this.isNotification(name);
         return (...args: any[]) => {
             const method = name.toString();
-            return this.rpcDeferred.promise.then(async (connection: RpcProtocol) => {
+            return this.sendWhenNoInit(async (connection: RpcProtocol) => {
                 if (isNotify) {
                     connection.sendNotification(method, args);
                 } else {
@@ -79,6 +88,10 @@ export class ClientProxyHandler<T extends object> implements ProxyHandler<T> {
         };
     }
 
+    private sendWhenNoInit(send: (connection: RpcProtocol) => Promise<any>): Promise<any> {
+        return this.proxySynchronizer.pendingProxyInitializations().then(() => this.rpcDeferred.promise.then(send));
+    }
+
     /**
      * Return whether the given property represents a notification. If true,
      * the promise returned from the invocation will resolve immediately to `undefined`
diff --git a/packages/plugin-ext/src/common/rpc-protocol.ts b/packages/plugin-ext/src/common/rpc-protocol.ts
index 107c639ef2c60..6f2a8792437b8 100644
--- a/packages/plugin-ext/src/common/rpc-protocol.ts
+++ b/packages/plugin-ext/src/common/rpc-protocol.ts
@@ -27,11 +27,12 @@ import { Emitter, Event } from '@theia/core/lib/common/event';
 import { ChannelMultiplexer, MessageProvider } from '@theia/core/lib/common/message-rpc/channel';
 import { MsgPackMessageDecoder, MsgPackMessageEncoder } from '@theia/core/lib/common/message-rpc/rpc-message-encoder';
 import { Uint8ArrayReadBuffer, Uint8ArrayWriteBuffer } from '@theia/core/lib/common/message-rpc/uint8-array-message-buffer';
-import { ClientProxyHandler, RpcInvocationHandler } from './proxy-handler';
+import { ClientProxyHandler, ProxySynchronizer, RpcInvocationHandler } from './proxy-handler';
 import { MsgPackExtensionManager } from '@theia/core/lib/common/message-rpc/msg-pack-extension-manager';
 import { URI as VSCodeURI } from '@theia/core/shared/vscode-uri';
 import { BinaryBuffer } from '@theia/core/lib/common/buffer';
 import { Range, Position } from '../plugin/types-impl';
+import { Deferred } from '@theia/core/lib/common/promise-util';
 
 export interface MessageConnection {
     send(msg: string): void;
@@ -83,6 +84,7 @@ export class RPCProtocolImpl implements RPCProtocol {
     private readonly multiplexer: ChannelMultiplexer;
     private readonly encoder = new MsgPackMessageEncoder();
     private readonly decoder = new MsgPackMessageDecoder();
+    private readonly initCallback: ProxySynchronizer;
 
     private readonly toDispose = new DisposableCollection(
         Disposable.create(() => { /* mark as no disposed */ })
@@ -91,6 +93,7 @@ export class RPCProtocolImpl implements RPCProtocol {
     constructor(channel: Channel) {
         this.toDispose.push(this.multiplexer = new ChannelMultiplexer(new BatchingChannel(channel)));
         this.toDispose.push(Disposable.create(() => this.proxies.clear()));
+        this.initCallback = new ProxySynchronizerImpl();
     }
 
     dispose(): void {
@@ -114,7 +117,9 @@ export class RPCProtocolImpl implements RPCProtocol {
     }
 
     protected createProxy<T>(proxyId: string): T {
-        const handler = new ClientProxyHandler({ id: proxyId, encoder: this.encoder, decoder: this.decoder, channelProvider: () => this.multiplexer.open(proxyId) });
+        const handler = new ClientProxyHandler({
+            id: proxyId, encoder: this.encoder, decoder: this.decoder, channelProvider: () => this.multiplexer.open(proxyId), proxySynchronizer: this.initCallback
+        });
         return new Proxy(Object.create(null), handler);
     }
 
@@ -149,6 +154,39 @@ export class RPCProtocolImpl implements RPCProtocol {
     }
 }
 
+export class ProxySynchronizerImpl implements ProxySynchronizer {
+
+    private readonly runningInitializations = new Set<string>();
+
+    private _pendingProxyInitializations: Deferred<void>;
+
+    constructor() {
+        this._pendingProxyInitializations = new Deferred();
+        /* after creation no init is active */
+        this._pendingProxyInitializations.resolve();
+    }
+
+    startProxyInitialization(id: string, init: Promise<void>): void {
+        if (this.runningInitializations.size === 0) {
+            this._pendingProxyInitializations = new Deferred();
+        }
+        init.then(() => this.finishedProxyInitialization(id));
+        this.runningInitializations.add(id);
+    }
+
+    protected finishedProxyInitialization(id: string): void {
+        this.runningInitializations.delete(id);
+        if (this.runningInitializations.size === 0) {
+            this._pendingProxyInitializations.resolve();
+        }
+    }
+
+    pendingProxyInitializations(): Promise<void> {
+        return this._pendingProxyInitializations.promise;
+    }
+
+}
+
 /**
  * Wraps and underlying channel to send/receive multiple messages in one go:
  *  - multiple messages to be sent from one stack get sent in bulk at `process.nextTick`.
diff --git a/packages/plugin-ext/src/plugin/documents.ts b/packages/plugin-ext/src/plugin/documents.ts
index 3eee7ade3a3d6..dc6bbbefe3ce6 100644
--- a/packages/plugin-ext/src/plugin/documents.ts
+++ b/packages/plugin-ext/src/plugin/documents.ts
@@ -158,7 +158,7 @@ export class DocumentsExtImpl implements DocumentsExt {
         const uriString = uri.toString();
         const data = this.editorsAndDocuments.getDocument(uriString);
         if (!data) {
-            throw new Error('unknown document');
+            throw new Error('unknown document: ' + uriString);
         }
         data.acceptIsDirty(isDirty);
         this._onDidChangeDocument.fire({
@@ -172,7 +172,7 @@ export class DocumentsExtImpl implements DocumentsExt {
         const uriString = uri.toString();
         const data = this.editorsAndDocuments.getDocument(uriString);
         if (!data) {
-            throw new Error('unknown document');
+            throw new Error('unknown document: ' + uriString);
         }
         data.acceptIsDirty(isDirty);
         data.onEvents(e);