I'm experiencing the same problem. Recently I've switched from MW 1.34 to 1.35. Visual Editor works fine everywhere except Flow. I'm using the autoconfigure mode, so $wgVirtualRestConfig is not set. At the moment I found three different error messages:
(1) When I'm tring to edit an old existing comment:
An error occurred. The error message received was: internal_api_error_Flow\Exception\WikitextException
(2) When I'm tring to save a comment in visual mode:
[X6akA72QQzSDKWPFYEuVSAAAAAU] Exception caught: Conversion from 'html' to 'wikitext' was requested, but core's Parser only supports 'wikitext' to 'html' conversion
(3) When I'm trying to switch from visual to text mode (the other direction works):
Unable to transfer content: Error contacting the server for conversion between wikitext and HTML. Please check your Internet connection or try again later if the problem persists. If you still get this error please file a bug
I just upgraded to MW 1.35.1 and encountered the same issue. I checked and the patch you mentioned (phab:T260648#6645078) was not included in 1.35.1.:(
But I was able to apply the patch, and it worked! Though I do hope WMF includes it in the next upgrade… I'm afraid it'll get overwritten when our staff upgrades our wiki next. They may not know how to fix this issue and ditch mediawiki altogether. It's somewhat frustrating that out-of-the-box fresh installs are this difficult to get working.
Anyhoo, thanks for mentioning the patch! I'm really glad it worked.