I have the same or a similar problem here. Some libs I can not delete. Some I can. After the first try (wich keeps the library in the list) I get additional the message, that I can't delete the library. The db is still in the attachment-path, but if I link the (not deleted) library to a project it is marked as not available afterwards.
Strange.
In any reason the deleting process on the server fails and the bim server does not mention it.
The library moved to another server and source path worked quiet well. The library duplicated and added by a new name failes too by deleting.
There is a logfile of the library/attachment activities, which shows, that it does not work, put the meassage there is crypted.