I"m making my log file available to you, via my OneDrive account (https://1drv.ms/u/s!AmYiifRfgrSrihicBQXoY0Rntse2?e=HaM8HX), as file size exceeds the max. allowable limit.
When you extract the ZIP file to the text log file, you will see an account of what happened, wile I was in the "Importing Metadata" phase of ingesting 340K files into my new database. The Notification Panel flashed me a message, to the effect that the Exif Tool was running into problems, and hence, needing to continually be restarted. Does this log file tell you why?
ExifTool hung only once and IMatch has to terminate it.
This can happen if badly corrupted files are processed. Search the log for E> to find the file names processed at the time.
Force a refresh of these files if metadata was not imported correctly.