1. You shouldn't open the central file directly! The only reason for opening the central file, if you want to create a new central file with save as. If you just double click on the central file in file explorer, it will open the central not creating a local, so this is one of the reasons you should always use the open dialog inside Revit. Always make sure, that create new local is selected on the open dialog, otherwise it will open the central directly.

You can easily check if revit created a local file, the local file should show up in your Documents folder, or the folder you set up in options>File locations>Default path for user files.

2. Possible reasons for you get that error message:
- Someone else is currently syncing
- Problem with your network, or hard drive in the server
- Two people using the central file with the same username

I am not opening the central file intended!
The option to create a local file is activated - nevertheless the central file will be opened...

For the secone Problem I can exclude the three Options you gave me, sorry

Did you try saving it as a new central?
Did you try audit?
Try saving it as detached without worksets than saving it as a new central.
Does this happen with all central files or only one of them?
Does this happen an all computers or just one of them, others can still open it and work on the file?
Everyone uses the exact same version of revit (e.g. 2019.2)?
How the shared drive mounted? With ip address or with domain name? Is it mounted the same way on all computers?

Also checkout Dan's post about problems with worksharing, try his tips: https://bimchapters.blogspot.com/201...d-central.html

If the local file box is checked, you are opening a local file based on the central file and are technically "connected" to the primary central file. If this is not what you want to do, you detach from central so that you are not working in the Central file but just a copy of it.