Hi Tom!
There is probably a small error in one of the qso records.

It might be a UTF8 character in name or qth tag. As UTF8 characters like ÖÄ é etc. have two or more bytes per character the adif tag's length number does not correspond bytes in adif tag string.

Or maybe there is an external character outside of any tag. (it can be unvisible).

Another error might be wrong frequency (out of bands), but I think this should make just an import error, not halt the whole process.

I have cleared out same kind of files by first checking from log what was the last qso properly imported.
Then opened the adif file with text editor and using "find" located the line where the qso is (ends to <EOR>).
Looked at the NEXT qso record if there is something weird and fixed it , or if nothing visible have not been found removed whole qso record and tried again to import.

Usually those files have had several (less than 10) qso records that have halted the adif import.
You just have to manually dig the all error records out and either fix or remove those records.