The method of locating the user file is stored within the company data directory, as are the security group files themselves. If you find that some companies work with a particular user ID and password, but the same ID and password does not work for a specific company, then either the security group files are preventing access to the user, or the path has somehow changed and cannot find the user file.
Neither of these scenarios would happen spontaneously, so something would have had to change in order to have access denied to a company in this manner.
Can you access the company data as the SYS user? If so, then look at the User setup and the Security Group setup (both from the File menu) to see if the list of users and groups are complete. If you don't see the correct list of users, the method of locating the list is looking in the wrong place. If the users are fine but the groups are not complete, then that data file has a problem and you would have to re-enter the groups.
If the SYS user is unable to log into this company, then the user list cannot be located for the same reason as above, or the password for SYS may have been changed in your office.
Because the question is related to security, it is sensitive to post more details on a public support forum.
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Regards,
Softrak Tech Support