Great that means that there is no problems with the database itself,
There are now a few things that need to be checked and there are a few conflicting things in what im seeing,
Part 1:
The corruption that you are seeing is normally related to the codepage not recognizing the characters of different languages, for example if you try and view something written in Mandarin on a PC setup for English you will see that sort of corruption of the characters, this would explain what we see except for the fact that you are also seeing values like "APPLIES-TO ENTRY" and that has nothing to do with a user id in a different language
It is still worth checking, does the NAV server, SQL server and Active Directory servers all share the same language and setup, and the user names were not created in a different character set?
Secondly does the database have a collation that matches the languages used in the servers
Part 2:
I am unsure of how NAV actually retrieves the users from the AD but it normally has a reliance on views within the database, for this reason running a NAV native backup and restoring it into a new database may be a solution, the reason for this is that NAV re-creates all the structural parts of the system and if any of these are causing the issue the re-build could do the trick
So create a native backup from within the classic client, create a new Database on the SQL server with a different name from within the NAV client, then restore the backup into it and check if you still see the issue in this test system
Note that this will not fix any of the transactions it will only affect newly created transactions, so in the test make sure you are creating new transactions to test with
Cheers
Nev