1) I see where you're coming from re Modal form
2) Yes, if you could leave search active then for weird cases like mine its easily fixed (see you're so good at plugins it does something you didn't plan! ;-) )
3)OK, here's what I did before getting error - dunno if recreatable.....
John Doe & Jr.
Tom Smith & Iii
Authors Jr. and Iii are considered "Similar"
I Close plugin
I type Author:"Jr." or Author:"Iii" into search
I realise there are a few where the suffix has been taken as surname
I open Tag Browser - Manage Authors - search for Jr. and then Iii
In both cases I "Find" all cases and ensure the Author and Author Sort are corrected.
I close Manage Authors and rerun Find Duplicates - Metadata
Authors - Similar.... Search - boom error, close Calibre, reopen, works fine for every type of search on every item.
4) My "does nothing" wasn't slagging off plugin - its my bad shorthand for "Didn't produce any results i.e. I had no "Similar" authors, but I have loads Soundex and Fuzzy Authors :-)
5) In Find Duplicates you can set a length to the Soundex - that was what I meant by tweak in Metadata :-)
Keep up the amazing work :-)