On testing further, I was able to isolate the MACTF file issue, which appears to be generated by Excel and/or macOS for xlsx files, possibly related to the AutoRecover/AutoSave processes. Even when disabling all of those, and saving as xls instead of xlsx, I still get a similar error -- but it ends up being far simpler -- Excel cannot access /Volumes/ -- plain and simple. My current hypothesis is that Excel has a (likely long-antiquated) issue with saving to volumes that have brackets in their name -- and Parallels creates its shared volume on the Mac side with the legacy Windows drive letter in brackets -- "[C] Windows 11" in my case. You can see one aspect of this issue if you try to save an Excel file with brackets in the name -- Excel says no, although Word is fine with it. I can't seem to find anyway to change the naming convention Parallels uses when it mounts Windows drive images on the Mac side as a shared volume. There is a setting in the Parallels Control Center, under Options and then Sharing, to "Assign a drive letter to shared folders" which is checked by default. I unchecked it -- but based on where it appears (under Share Mac), it is for when you're mapping shared folders from the Mac side to Windows, not your actual local hard drive image or vice versa. I couldn't find any parallel (no pun intended) setting for omitting the drive letter for local drive(s) from the Windows side to Mac.