OK, found the problem. PS 22 now requires more advanced GPU support. By default the GPU is turned on, and because my system does not meet the minimum requirements, opening any file causes the IO Error. I tried turning OFF Use Graphics Processor in the Preferences and it could then open files. But attempting to close down the program failed with the same error. After killing it from Task Manager the changed GPU setting had reverted to ON - a vicious circle. After several attempts to solve this it now seems to be working (with GPU off). Ridiculous to break a previously working program with new hardware requirements.