'Excel not showing errors in spreadsheet

I have two PCs, both have Office 365 installed (Build 16.0.14827.20180, 32bit).
Both load the same Excel sheet from the network.
First PC shows the error correction correctly, cells are marked with a little green triangle on the left top.
Second PC, with exact the same Excel settings as the first, does not show/mark those errors ...
It only happens with that one file, using another file or just doing a fresh spreadsheet shows the errors.
I reseted the "ignored errors" in options, didn't help.

An idea(s) guys?

Thanks, Tom



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