Please try the following steps:
1. Close Connexion client completely for yourself and all other users.
2. Open the folder %appdata%\Roaming\OCLC\Connex\Db.
a. Type Windows Key + R.
b. Enter "%appdata%" into the field and click OK. This will place you in the Roaming folder.
3. Make sure that file extensions are fully displayed using the View > File Name Extensions menu option.
4. Check for the file extension .ldb. Files with this extension are what tell the Access database running Connexion client that someone is logged in.
5. Delete this file and attempt your workflow again.
Further error message: Unable to open file in exclusive mode.