A 3125 error = the OS locked the files that Muse needs
there are many ways to cause it from anti-virus, wrong network setup i.e, dropbox, and a system crash can all lock the files
to be clear, your system has other issues and these are just work arounds to get you back to work until you fix your system
Please help us with little more info for digging into your issue!
1) from where are you accessing your files i.e. any network drive or local hard disk.
2) Please check if you have an apostrophe in your system's username or startup disk name? Muse gets confused with the apostrophe!