Random 403 errors

In the company I work for, we use Gitlab for hosting our projects for about 5 years. For a few weeks we have random 403 errors for some people at random moments.
When we change the internet connexion (from WIFI to 4G for example), it often fixes the problem, but not always.
When the problem occurs, it occurs for all the repositories hosted by GitLab, but no problem on Github for example.
3 people have regularly the problem (20 people in the company). We use Windows but not necessary the same tools (SourceTree, command line).
After a random time (few hours), the 403 error stops.