Just for the record, I edited my .metadata.calibre file and replaced there
with
(means I also deleted all comments from this file), after that the file was 121000 KB, but unfortunately no change with the error.
What I did not mention so far, there is one thing that nearly always helps: Kill Firefox.
After that everything works, even with the new file without the comments.
I just hoped I could connect to my folder without having to kill Firefox first...
Is there another way to connect my library to a folder in a way that Calibre remembers which book in the folder belongs to which book in the library and can update the folder by writing books there in "{author_sort[0]}/{authors}/{title}" form?
What I don't need with this folder are any kind of metadata info...