Take a look through your tracks to see if any of the track "Edit Filters" are set to "Transients". The edit filter is that dropdown menu on the tracks where you can switch from "Clips, Automation and Transients".
If any are set to "Transients" then that might be causing it.
If you have Groove Clips in the project or open Loop Constructor things going on I think that would make some transient crap happen.
This is all total guesswork. I do not know for sure but the assumption I am making is the message/hold up is due to Sonar/the computer trying to make a list of all active/open transient markers in the project and their position/status/whether they have been moved/etc.
In X2 I had a BIZNOTCH of a time while trying to close/save/reopen with open audiosnap clips and I think that was the issue (it led to serious crashes and project corruption). As far as I can remember I don't THINK I got that specific error message but that was years ago... and I may have actually gotten it.
Point is since then I try not keep any active transient gack open when closing a project. I do the work and bounce it even if I have to do it all one section at a time.
Pretty sure things like Vocal Sync and Melodyne tap into these transient data aspects of the program too.
Again... total guesswork and n00b/non programmer understanding of the issue BUT maybe it will help point you to a solution/answer OR might jog the memories of the smarter folks around here.
Cheers... and good luck.