I had this exact same problem loading MULTIs too, way back in the SampleTank 2XL days. I never solved the problem though. At that time Tech Support said to try reloading everything, which I did try, but it did not solve the error. Everything else about ST worked except the MULTIs loading part.
I wish there was a way for the system to provide a log of some sort after an error has occurred, so we could get to the bottom of this kind of problem. It is hard to figure out where to start looking without more details. Could have been a missing file, could have been a file or folder name inconsistency, could have been a drive letter inconsistency, could have been an I18N glitch, could have been a corrupt file, who knows.
I like the idea of MULTI's as a quick way to set up a group of instruments, but in lieu of the errors precluding it working consistently, have had to revert to setting up instruments in the DAW and saving the DAW projects as templates. Not ideal.
https://www.ikmultimedia.com/contact-support/sw/may know more about the current state of affairs, though.
HTH