Check your EXCEL source spreadsheet that was used for import - it likely contains ''illegal'' characters in your test names:
the usual offenders in names of tests:
~'!@#$%^&*()+=|\}]{[''':;?/*<>;.,

sometimes, heavily marked up/formatted text can mess things up (like odd bullet characters).
Also, if people from other character set countries use copy/paste of various text, it could also carry across ''bad'' characters that QC cannot resolve.

The GUI/form usually catches this, but when importing from EXCEL with the Excel Add-in, it sometimes lets illegal/bad characters in.

In your case, I would focus on looking for illegal characters in the NAME of the Test or Test Steps.

If you can, delete one-by one, the tests that are blank, starting at the TOP of the list, then refresh to see if it was the one causing the issue.
You could also go into the project database (make a backup first), and then into the TESTS table, then look for the newly imported tests and examine their names for bad characters (you may already know which one is bad from examining raw EXCEL source data) - then you can RENAME in the DB table to a good name.

When your users get out and back in to the project, it should work fine again.