As near as I can tell, converting a BeautifulSoup4 (4.10.0 to be exact) soup back to a text (str(soup)) in Calibre 6.10.0 some how crashes out instead of raising the appropriate RecursionError.
But trying it in Calibre 5.44 (BS4 version:4.9.1)
does raise the error appropriately.
I forced my CLI python install to BeautifulSoup4 v4.10.0 and it worked as expected, raising RecursionError, suggesting it's something Calibre specific, not BS4.10.
This is not, apparently, limited to Calibre background job procs either. Running it in the foreground will also
immediately cause the Calibre GUI to exit without any warning or details, even in debug mode.
I've opened a calibre bug(
2000888) to bring this to Kovid's attention.