Many times when we see this error it is because one of the fields on the test case has an ''illegal'' value in it.
One of a few things could probably be causing it:
Most Likely, It could be that you have recently changed the field type on one of the test fields in ALM, lets say for example the field ''priority'' was originally of type ''string'' in ALM configuration then you decide to change the field type to ''date'' so that the field won't accept text values, lets say that previously a user had put in the value of ''Urgent-1'' you would then receive the error that you are getting since the value ''urgent-1'' is a string and not a date and the field now only accepts values of type ''date'' More than likely this is what has probably happened, I would find out if anyone has changed an ALM field type recently
It could also be a special character, or something a user copied and pasted from an email, word document, etc into a field and ALM does not like the special character or value.
To fix this you would have to identify what field was changed in ALM or which field contains the ''bad'' value in the test, you could either go through and clear the value for the field in that test (directly in the database since the UI won't let you see the values). Alternatively you could switch the problem field type back to what it was originally, then clear all the values and change the field back to the type that you want it to be.
The best way to figure out if this is what is happening is to look in ALM customization and make a note of what type each test field is, then go in to the ALM database and query the database for one of the test ids experiencing the issue (should be in the tests table for that project) then see what values are put into each field and make sure those match up with the field types.