You mention exporting Articles and Issues, but importing Users.
Are you exporting Users from the old OJS system and into the new OJS system?
If so, my next step would be to try to identify the data that was causing the problem. You might look at the original system for the longest mailing address, or you might try exporting/importing only half of the users to see in which half the problematic user lives.
Without seeing your data, one other possibility to consider would be whether you are changing character sets in the databases. If you export Unicode data from one database and try to import it into a non-Unicode aware database, it may appear to the non-Unicode database to be significantly longer than than it really is.