"Error uploading the opml file" - but W3C feed validation service says "This is a valid OPML feed"

[SprickW]

OK, it also says:

This feed is valid, but interoperability with the widest range of feed readers could be improved by implementing the following recommendations.

followed by

An element with more than just a "text" attribute should have a "type" attribute indicating how the other attributes are to be interpreted.

The W3C validator you recommended marks the closing XML-element of

The error message doesn't indicate where to error happens, so after quite some time trying to fix the problem I gave up and hope that you may give some hints how to fix the problem.

OPML files where exported by Newsflash and Liferea, I consider them to be in widespread use.

[Grotax]

Assuming you are using the latest version of News you will find the reason why a feed could not be imported in the logs from Nextcloud in the admin section, if not you could update news and try again, feeds already imported or feeds that can't be imported should be skipped and the errors should be in the logs.

The issue with RSS and Atom in general is that some feed-generators do not stick to the standards, the library that we use to parse the feeds "feed-io" can handle some stuff but it is not possible to handle everything.