>When I try to add or create a new custom dictionary or create a new custom dictionary, Word crashes<
I had this same problem with Word 2021 version 2403 Build 16.0.17425.20176 on my primary development box. I tried a number of different workarounds such as creating the new dictionary in the protected user profile AppData directory, creating it in non-protected area of a 2nd disk drive, etc. Word would accept the browse location but then would crash with a 0xc0000005 / Access Violation error in MSO.dll after clicking "Save". This box has a very large number of installed Dev tools, word processing tools, graphics tools, CAD tools, foreign language apps, etc. So it has very large user Profile, very large Registry hives, very large extended Path, etc.
So based upon thread replies here, I also tried creating a custom dictionary on my backup box which is much newer and has never had even 5% of the software installs done on it. By comparison the backup box's AppData folder, Path length, Registry size, etc. are all just a tiny fraction of the size of those on the primary box. But I still got the same A/V error in MSO.dll when clicking to Save the new dictionary; again without regard to whether or not the new dictionary was pointed at the User Profile AppData folders or at folders on a separate disk drive.
I studied the Custom Dictionary dialog and noticed that it has a "Dictionary language" drop-down box that defaults to "All languages". Still working with my backup box - I first restarted Word to get the situation as clean as possible; then selected "English (United States") on that "Dictionary language' before further testing.
The next time I tried - still on my backup box - to create a Custom Dictionary in the default AppData profile folder, there was no error and the process completed quickly and successfully. I then also tried creating a 2nd Custom Dictionary in a non-User Profile directory on a separate "data" disk drive. That also succeeded without incident.
I returned to my primary Dev box and repeated both tests (one new dictionary in my user profile folder structure and a second on a separate drive) with equal, uneventful success.
Hope this helps anyone else seeing this error. Hopefully the MS Office Dev Team can use this information to help resolve the underlying problem when the default "All Languages" option remains selected.