bug with the use of excel shared file

Hello,

In our company we have Microsoft 365 for Business and Teams groups with Sharepoint

The small planning team in my company is using a shared excel file, placed in Sharepoint and that they open with a right-click on the link and opening Excel for Web. All was fine until a fourth user came and now once in a while the file is locked with a message (translated) "Someone has locked your file. A member is currently modifying this file with a version without license of Excel. To modify this file together, everyone must use Excel with Microsoft 365 or Excel for the Web".

Everybody is using Excel for the web, the licenses for M365 Business are paid, thank you, and the message comes to any of the four people working.

What is happening and how can I fix it ?

Thank You (and I have difficulties to keep the smile)