Just seen your edit after digging out some .fits example files.
Before re-installing Gimp. The file loaders are largely individual plugins so:
In Gimp: Edit -> Preferences -> Folders -> Plug-ins where there should be an entry pointing to your Gimp system plug-ins folder.
Typically C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins
You can check that location and files exists with Windows Explorer and if not add in the Preferences dialogue or try that Reset Button top right.
Otherwise, all your settings are held in your Gimp user profile C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\ rename that ...\2.10-saved Restart Gimp and it creates a new default profile, the same as when you first ran Gimp. Try Gimp with some files.
If that does not work, now is the time to consider a re-install.
...and .fits files work here. https://i.imgur.com/6RzESyd.mp4
Edit: Just a thought, all your files are local ? Nothing on a network / cloud / dropbox that might prevent loading ?
Before re-installing Gimp. The file loaders are largely individual plugins so:
In Gimp: Edit -> Preferences -> Folders -> Plug-ins where there should be an entry pointing to your Gimp system plug-ins folder.
Typically C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins
You can check that location and files exists with Windows Explorer and if not add in the Preferences dialogue or try that Reset Button top right.
Otherwise, all your settings are held in your Gimp user profile C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\ rename that ...\2.10-saved Restart Gimp and it creates a new default profile, the same as when you first ran Gimp. Try Gimp with some files.
If that does not work, now is the time to consider a re-install.
...and .fits files work here. https://i.imgur.com/6RzESyd.mp4
Edit: Just a thought, all your files are local ? Nothing on a network / cloud / dropbox that might prevent loading ?