And can you recreate it from a backup or should I create a new one?
As for the "not found" bug, the problem seems to be in querying calibre.zip: If called from a different directory without PATH defined, ebook-convert.exe queries not the archive itself, but a directory library.zip:
Code:
10:45:40 ebook-convert.e:2880 OPEN D:\Tmp\calibre\ebooks\lrf\output.pyo PATH NOT FOUND Options: Open Access: All
10:45:40 ebook-convert.e:2880 OPEN D:\Tmp\calibre\ebooks\lrf\output.pyo PATH NOT FOUND Options: Open Access: All
10:45:40 ebook-convert.e:2880 OPEN D:\Tmp\calibre\plugins\output.pyo NOT FOUND Options: Open Access: All
10:45:40 ebook-convert.e:2880 OPEN D:\Tmp\calibre\plugins\output.pyo NOT FOUND Options: Open Access: All
10:45:40 ebook-convert.e:2880 OPEN D:\Tmp\calibre\library.zip\output.pyo PATH NOT FOUND Options: Open Access: All
10:45:40 ebook-convert.e:2880 OPEN D:\Tmp\calibre\library.zip\output.pyo PATH NOT FOUND Options: Open Access: All
But the archive calibre.zip is never actually queried.
On the other hand, if I set the PATH, when all attempts above fail, calibre.zip is also checked for the existence of the file. That seems to me like a bug in file-locator rather than a Windows quirk with loading libraries.