The Launcher has a Data Mismatch Error

The launcher displaying a data mismatch error can appear due to mods, corrupt files, or unknown data. This can be from installed mod files, old data files from previous patches, or even your favorite image of Astarion you’ve decided to save in the games Data or bin folder. 
It is also important to note that if you have the Toolkit installed, this will cause a data mismatch warning that you can essentially disregard. 

To remove or identify what is casuing this warning, you can follow the following steps:

There should be a log file listing any files in the install folder which are not expected, as well as files in the Mods folder. Copy %LocalAppData%\Larian Studios\Launcher\Cache into the location bar in Explorer and hit Enter, then look for a text file named for the platform (steam/galaxy), the game version number and ending with '_alteredFiles.txt'.

There may be a problem with the patch update process leaving old patch hotfix files in the install folder, causing startup crashes. Check the '..\Baldurs Gate 3\Data' folder, and if there are any files named Patch#_HF#.pak, where the first # is anything less than the current patch, try deleting them. An easy way to get to the install folder :

For Steam, in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Browse...' button to open an Explorer window at the main install folder. For GoG, in Galaxy select the game, then the settings icon at the top right (beside the Play button) and under Manage Installation select 'Show folder'.

If you have installed mods, or have had mods in the past, check the 'Mods' and 'Public' folders in the '...\Baldurs Gate 3\Data' install folder; if either folder is present, they are safe to delete. If applicable, you can also delete the modsettings.lsx file in the profile folder, if you're not using a new profile for the current patch.

With patch 7, rather than My Documents, the game uses the local appdata folder for the current user account: 'C:\Users\<UserName>\AppData\Local\Larian Studios\Baldur's Gate 3'. The easiest way to get to the profile folders would be to copy %LocalAppData%\Larian Studios\Baldur's Gate 3\PlayerProfiles into the location bar in Explorer, hit Enter and browse down from there (or on the same level for the '..\Baldur's Gate 3\Mods' folder).

Try verifying local files; this will not affect extra or duplicate files, but if the data mismatch may be a result of a problem with the update or install process, it will check the files that are supposed to be there for anything missing or corrupt. 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 the game, then the settings icon at the top right (beside the Play button) and under Manage Installation select 'Verify / Repair'.

If you are still having an issue launching the game or in game, please contact Support. Especially if the game is crashing, please include a dxdiag report. Press WinKey-R, type in dxdiag and hit enter, then when it finishes loading click on the 'Save All Information...' button and save the report somewhere handy to then attach to your email to support. Also check the '...\Baldurs Gate 3\bin' folder and '...\bin\db\attachments' or “...bin\db\reports”, and if there are any CrashDump files being created, zip a few together to include.

If you are on Mac, you can use the 'System Report' button from the 'About This Mac' Apple menu item, or the 'System Information' app in the utilities folder, please also include the OS version number. Crash reports are located in the 'Console' application, under 'User Reports', if any are being created. Alternately, the crash reports may be saved in the '~/Library/Logs/DiagnosticReports' or '~/Library/Logs/CrashReporter' folders.