Some warnings when converting:

[mlevin77]

When converting, I see:

W, [2019-07-07T05:54:32.781197 #35256] WARN -- : Bad date/time value "0000:00:00 00:00:00": mon out of range

is it possible to have it tell me which file has the bad timestamp?

W, [2019-07-07T05:58:24.629657 #35256] WARN -- : Badly formed IFD: Infinity

what does this mean, and how to find out which photo it corresponds to?

When it finished, it said

One or more files were missing from your iPhoto library! See 'missing.log' in output directory.

 the missing.log lists a lot of files, but it seems to repeat most of the names again and again. Is that normal, and how to find the "missing" files?

[jensb]

These are printed not by iphoto2xmp, but by some library which is used.
I'd have to do some digging to find out which library this is.