Hi @Zoltan Gerenday ,
Since we can't reproduce your situation, we need to narrow down the problem and try some things.
- How do they connect to your internal SharePoint site? Perhaps you can confirm the next steps with them in detail.
- Does this error happen to everyone but you?
- Try to use your browser's InPrivate mode to see if the same error occurs.
- Are other users with permissions facing the same problem? View through another computer or account.
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