I had also received the unable to open file error. I rebooted the computer and that solved it. Note: Being previously in software development, I suspected that the file may still be opened and stuck in a 'zombie' state by windows and TTax won't open two instances of the same file. My files are saved on Microsoft's OneDrive cloud service which attempts to sync the file with their cloud server. Sometimes, I put my computer to 'sleep mode' immediately after closing TTax. My suspicion is that by not giving TTax time to fully close out before the computer goes to sleep, that issues could occur. Rebooting cleared that up.
