Cannot rename Vendor No. (NAV 2013 R2:Unable to convert)

Hi everyone,
we have the following problem. We cannot rename any Vendor No. in our production server. The same code is running in stage and development servers with no problem.

The error message from NAV is "Unable to convert from Microsoft.Dynamics.NAV.Runtime.NAVCode to System.DateTime."
The error is raised after the code in the Rename trigger in Vendor table is completed. So we do not have any indications of what is going on, because the debugger isn't stopped in any error.
Obviously, if we put a new field (e.g.an autoincrement) as Primary key in the vendor table we can modify the Vendor No
The problem exists even if we delete all vendors and create a new one with Vendor No. = 1.
We have rebuilt all indexes in the db, and make the known steps for db's maintenance, but with no positive result. ](*,) ](*,) ](*,)
Any proposal, solution or thought is welcomed.
Kind Regards