Looks like .lck file was accidentally removed from the system which Dreamweaver is trying to access. Please try the following steps to overcome the issue.
1. Create a new file
2. Copy & Paste the code from the old file.
3. Delete the old file locally and on remote server
4. Rename the new file to desired file name and check-in it to the remote server
5. Try with check-out and check-in for the new file
If you are still facing the issue let me know and if you can forward the file to firstname.lastname@example.org I can check if it is reproducible and issue is specific to the file or not.
Yalpi Shiva Prasad
Adobe Dreamweaver Team
That worked, thanks.