There are many issues that can occur with workflow within Microsoft Dynamics GP. The first step should always be to run the following script on your database (After a backup).
If this does not resolve your issue, then I would recommend checking your setup. Sometimes a manager will be removed or an email changed. I had one instance where workflow was being used and the email server was changed and this caused an error.
IF after you ran the script and went through the setup steps to verify that everything is correct, then I would open a case with Microsoft.