One option is to use Access Data checker resolve this issue.
**Please ensure you back up your database before following these steps below**
Open up Data Checker, and log into the relevant database. Then:
Click Upgrade data for Access Accounts
Then click Generate users from Accounts data
This will then run through the process as a pop-up, let that run out. Then the users will now be able to get back into Dimensions.
Another option, if the user has not yet used Dimensions, then you could try deleting the user and recreating them. You can delete a user as follows:
Ribbon style menu:
Select System, User Records from the main menu
Scroll to find the user you wish to delete
Click Delete
Module Menus / Classic style menu:
Select System Control, Users, User Records from the navigation menu
Scroll to find the user you wish to delete
Click Delete