It's either a left-behind semaphore or lacking file system permissions to update the ACDB (access.db) file.
If you use the VM File Server, login to its management web page (http://localhost:8080/serenafs/admin
- on the server) and see if there's a semaphore. If there is one, clear it from that interface. If there is no semaphore, make sure the user running the Serena VM Web Application Server process has permissions to update the ACDB file.
If you are directly accessing the file system, look for a *-sem or *.lck file in the folder where you have the ACDB file and delete it if found. If there is none, make sure the user running the operation can update the ACDB file on disk.