Try rename “SyncViews.3dm” and open Worksession. Then close WS and remove the file and try open WS again. Are you then out of option to detach?
I have another problem with Worksessions. if I open a file (part of WS) and start inserting a block, but then aborts the insertion before confirming the insertion, then I still see the Block’s layers show up in the file (not expected) but no block in the Block manager (as expected). And if I try to delete those layers it refuses due to a Block definition on the layers (which are not there, at least not visible in the block manager.
Solution: Close Worksession altogether and open the individual file, then it allows you to remove the aborted block insertion layers.
Perhaps this post should be split, but it seems to be a problem with Worksessions messing up files/references in runtime.