This is a problem with SANE rather than scanservjs. It usually signifies a problem with the driver. Your best bet is going back to first principles with SANE itself. Follow the steps here
Some scanners mis-report their size - don't know why, but they do. This means that when the app attempts to crop things the maths is all wrong. The best way around this is to override the reported scanner dimensions. See this recipe for more.
This happens when the browser received a string from the server which is not a valid JSON string. Most likely you're running a proxy server (e.g. nginx) which is timing out prior to the completion of the request. Scanservjs can sometimes take a little while to fulfil its requests - usually because it's waiting for a scanner, but sometimes because it's having to do a fair amount of image processing and you're running on a low power CPU (e.g. RPi).
The solution is to increase the proxy timeout. For nginx that might look like:
server{
...
proxy_read_timeout 300;
proxy_connect_timeout 300;
proxy_send_timeout 300;
...
}
When scanning files with high resolution, e.g. 1200dpi it is very likely for the
request to timeout. This is because node HTTP times out after 2 minutes by
default. The solution is to increase the default timeout. That's possible by
setting config.timeout = 600000; (for 10 minutes for example).