I got an answer by the support team :
[i]Here please find workarounds while we are waiting for the fix from Microsoft.
1. Rename the FEFLOW - MS office plugin. Please note you cannot use any office files in FEFLOW for this option.
You can use the below workaround to disable the Excel-Import feature to make FEFLOW operating again:
Open the file browser and rename the file: C:\Program Files\DHI\2021\FEFLOW 7.4\bin64\MSPlugin20.dll (accordingly to the bin64 folder for any other FEFLOW version)
e.g. MSPlugin20.dll -> MSPlugin20_backup.dll
If you wish to use Excel files still, they can be saved in another file format, e.g. as *.dat. This should allow the file to be imported even the plugin is not working.
2. Downgrade the MS office to a specific version that supports FEFLOW.
So far the tested version is MS office 2106 (Build 14131.20320, released on July 13). Please refer to Update History for Microsoft 365: https://docs.microsoft.com/en-us/officeupdates/update-history-microsoft365-apps-by-date
For your reference, here is a link about how to downgrade MS Office:https://support.microsoft.com/en-us/topic/how-to-revert-to-an-earlier-version-of-office-2bd5c457-a917-d57e-35a1-f709e3dda841