Is this with the respec mirror on the Lady vengeance, or when using a mod (workshop or from the gift bag menu) to add one to Fort Joy?
Does it matter if you try with different characters, if available?
If you start a new game (on PC create a new profile first to keep quick and auto saves from overwriting ones in your current playthrough) on story difficulty, skip through the ship section as much as possible to get to Fort Joy, enable the respec mirror mod in the gift bag options and check the mirror in the Fort Joy arena, does that work?
On console, starting the game from a different account would give a different drive space for saves.
Alternately, if you don't already have an honour mode difficulty save, starting one would avoid overwriting saves, since it uses a separate, single save slot (in the difficulty list, after selecting Tactician there is a choice between Regular and Honour).
If you are playing on PC, try verifying local files, to see if that helps, or on console try clearing the cache.
In the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Files...' button.
With the GOG version, in the (optional) Galaxy client, select Divinity: Original Sin 2 in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'. In Galaxy 2.0, select the game, then the settings icon at the top right (beside the Play button) and under Manage Installation select 'Verify / Repair'.
With the App Store release, or GOG in OSX without the Galaxy client:
Terminal
cd ~/Library/Application\ Support\ ... up to the Divinity Original Sin 2.app folder
codesign -vvv Divinity\ -\ Original\ Sin\ 2.app
For the first command, it may be easier to browse to the install folder in Finder, then drag and drop the location into an open Terminal window (though for the App Store version the default install path is just in /Applications ).
The second command takes a while, and will output some lines starting with '--prepared' or '--validated'. If everything is fine, it will end with:
Divinity - Original Sin 2.app: valid on disk
Divinity - Original Sin 2.app: satisfies its Designated Requirement
To clear the PS4 cache: press and hold the PlayStation button on the controller, select 'Turn Off PS4', wait another 30 seconds after it finishes shutting down, and start it again.
On Xbox, to clear the cache:
- press and hold the Xbox button on the front of the console for 10 seconds until it shuts down
- unplug the power cable and wait 30 seconds
- plug the console back in and turn it on
- log into the same Xbox Live account you were using, start the game and let it sync, if required
On Switch, in the Home menu open System Settings, then select System, Formatting Options, Clear Cache and then the current user.
Note that this will also reset the system language and brightness setting.
If that doesn't help and you are playing on PC, please reply with your latest save (or a Dropbox or Google drive link, etc). If you are using any non-Larian mods, also include a list of those enabled and/or the modsettings.lsx file from your profile folder.
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).
PS4 saves can be sent, as well, but they would need to be transferred to a USB flash drive first.
In the Saved Data in System Storage section of the Settings menu, click Copy to USB Storage Device, select the relevant save(s) and Copy. From there you can zip the files, then copy to email or upload somewhere and provide a link.