You can try out the below steps to troubleshot this:
Troubleshooting Approach 1
Step 1: Run CSISYNCCLIENT.EXE
Using file explorer, open the CSISYNCCLIENT.EXE file found at the following address.
C:\Program Files (x86)\Microsoft Office\root\VFS\ProgramFilesCommonX86\Microsoft Shared\OFFICE16
Right click and select 'Run as Administrator'.
Step 2: Verify
Verify the CSISYNCCLIENT.EXE is running by opening task manager and looking for 'CSISYNCCLIENT.EXE' in the process list. If it is not running try once again to 'Run as Administrator'.
Step 3: Restart PC
Once the process is verified as running, restart the PC.
How to: The server you are trying to access is using an authentication protocol not supported by this version of Office
[FIX] The Server You Are Trying To Access Is Using An Authentication Protocol Not Supported By This Version Of Office
Troubleshooting Approach 2
You may try these troubleshooting steps regarding the error you encountered:
Clear Windows Credential:
- On the Control Panel, go to User Accounts > Credential Manager >
Manage Windows Credentials.
- Delete all MicrosoftOffice15/MicrosoftOffice16 credentials.
Open Word/Excel application then go to File > Accounts. Sign out the current account and then login the Office 365 account with a valid OneDrive for Business license.
- Clear OneDrive for Business cache using Easy Fix Tool:
More on this troubleshooting in the below thread:
The server you are trying to access is using an authentication protocol not supported by this version of Office
- By default if we are in VPN connection, we cannot connect to SharePoint online, first we need to disconnect from VPN, then login in SharePoint online, then again re-connect the VPN back, then this will continue to work as expected. This is the default behavior of VPN with respect to SharePoint online.