File upload - file could not be upoaded. Please try again uploading it again validation message

Hello,

We have a start form with a simple file upload on a cloud installation, Version 21.4;

Some users experience for some files the error from attached print screen (had to hide file name); Files have .tx extension.
I can't receive the files as they contain business sensitive data so we cannot reproduce error on lower environments.
Checked in logs and couldn't find any clue. 
In admin console, under file upload we don't have any extension blocked just the check for "Enable Real-Time Virus Scanning", is it safe to disable this and ask user to re-upload same file? in order to figure if it's from this one?

Older similar post doesn't offer any solution, it's not the case for VPN now, /logs/audit/blocked_files.csv is empty in my case as well.
https://community.appian.com/discussions/f/new-to-appian/18528/certain-pdf-documents-are-uploaded-but-not-others-message-file-pdf-could-not-be-uploaded-please-try-uploading-it-again/73058#73058

If you have experienced something similar please let me know how to investigate or at least replicate this issue without having access to files that cause the problem.

Regards,
Mihai.