ERR_CONNECTION_RESET on SOME website, but only on Windows clients

[Jean-Christophe]

Hi,
I moved from Sophos UTM to OPNSense a few days ago.
Since the migration, a few websites (always the same) give me ERR_CONNECTION_RESET in the browser.
For some of them, if I let the error displayed, after a few seconds, it refreshes and it works. Not all of them act like that.
I looked at the firewall logs, I don't see any error.
Interestingly, it only happens on Windows computers (any browser).
Linux and Mac look like they are spared by the problem.

Any help would be appreciated.

Have a good day,
JC