@Peter82: I was talking about the Python version Calibre runs on (i.e., 2.7 for Calibre 4, 3.x for Calibre 5), not the one used for LibrarianSync on the Kindle, which is irrelevant here

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I vaguely recall that's there's a button somewhere in the GUI to restart in Debug mode, I'm sure it's detailed in the FAQ or has been mentioned approximately seven billion times by plugin devs in this very forum

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(I happen not to remember it right now, because I mostly just run
calibre-debug -g in a shell myself).
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Thanks for confirming that I apparently fixed it without breaking anything else ^^.