Since I was asked in game and didn’t remember all the details. Here are details for how to replace the individual files from backup (rather than the full folder as explained in the linked Help article).
To identify a character, first log in using only that single Eve client, log all the way in not just character select. Then log out again, this will save the settings file and last modified is the one you are looking for.
Close the launcher.
All your settings are in %LocalAppData%\CCP\EVE\g_games_eve_sharedcache_tq_tranquility
In here you have a folder for each profile, possible just settings_Default, go into the folder used by the account in question and sort by date modified.
You want the core_user_* file (not core_char), the user number here is your account number, and as far as I know secret so don’t share it.
To keep the existing file as backup to be able to undo the change if something breaks, fx add .brokenNeocom to the end of the file.
Now take the core_user from your backup or Repeat to locate the core_user file for a working character, and copy this to the folder with the .brokenNeocom file. Rename the copied file to the original name of the .brokenNeocom and it should now work.
Notice that this is all done without editing any of the files, and will possible cause many more changes depending on age of backup / difference on characters.
For more details on how to sync changes between characters etc see Manually copy settings between characters and accounts