Hey, I finally moved to new programming software this week (WingIDE to PyCharm). It was a little jarring dealing with new UI and shortcuts and a billion new settings, so I mostly stuck to simple code cleanup to get to grips with it. I don't have much exciting in my changelog beyond a tweak to system:hash, so rather than put out a thin build tomorrow, I will do some more work instead and move the release on a week.
If you haven't seen it, though, please check out the new help a user put together: https://hydrusnetwork.github.io/hydrus/
It has nice features like search and tables of contents and will be easier to edit in future. 475 will have it too for the local copy and should be out on the 2nd of March.
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If you check your 'client - date.log' file in the install_dir/db folder, and go to the bottom and look for the timestamp when this happened (and maybe ctrl+f for "400" or "unknown error", whatever text you have), can you see any 'traceback' information for the errors here? Can you clip and pastebin them or just post here? That will show more information on what is actually failing here.
If you are getting popups in the client UI, it should let you just copy the traceback there, too.