Able to merge contact records except for 2

Hi There,

An end user is able to merge contact records with no issues or problems except for 2 contact records.  When she attempts to merge 2 contact records, she receives this error message -

The download log file button is not available so information is extremely limited.  I could, as an Admin, merge the records, but that's not the point.  I'm trying to figure out why these 2 particular contact records are throwing the error while all other contact records are able to be merged.  The records are both active, she is the owner of both records, I can see no major differences between them.

Thoughts anyone?

Thanks in advance!