bus paths suddenly becoming unavailable

[right beside]

i'm having a problem where more and more bus paths are progressively becoming 'unavailable', even though they are active in my i/o

after doing some work, i saved and closed a session. later, when i opened it up again, some of my bus paths were greyed out. I tried to activate them and got: the output could not be made active because the output path is currently not available

checked my i/o and the paths are still there. wasted a bunch of time deleting paths and re-importing. it worked. but when i saved, closed and re-opened, the same paths were greyed out again. LET ME REPEAT: all were working when i saved the session. but upon opening it moments later, the paths are 'unavailable'

what's worse, is that the issue is spreading. each time i open it, more and more paths are 'unavailable'.

i even did a 'save session copy' onto another drive. opened the file there and the same problem is there.

so far, no other sessions are affected. just this particular one. if i open an older session file or an older backup, the problem is there too.

does anyone have any idea what is going on?