I have had this problem a few times now and it seems to affect sub assemblies that are mirrored using "opposite version" option.

When opening the assembly an open file window pops up asking for a particular part it apparently cant find even though it exists, you select the part and click open and the window just pops up again and will continue to do this infinitum. If you click cancel the assembly will open but with the file suppressed, if you attempt to unsuppress it will open the "open file" window again.

Any ideas? this seems to be a bug.