No email and warning message

[#1]

No access to emails here for around 6 hours or so.  I get the following

Your connection isn't private

Attackers might be trying to steal your information from webmail.plus.net (for example, passwords, messages, or credit cards).

net::ERR_CERT_COMMON_NAME_INVALID

[#2]

It says it's the Plusnet webmail domain that's at risk, not my computer or browser.

This is trying to access my emails via web browser as I always do.

On clicking 'advanced' I get:

webmail.plus.net uses encryption to protect your information. When Microsoft Edge tried to connect to webmail.plus.net this time, the website sent back unusual and incorrect credentials. This may happen when an attacker is trying to pretend to be webmail.plus.net, or a WiFi sign-in screen has interrupted the connection. Your information is still secure because Microsoft Edge stopped the connection before any data was exchanged.

You can't visit webmail.plus.net at the moment because the website uses HSTS. Network errors and attacks are usually temporary, so this page will probably work later.

[#3]

Same happens in Chrome.  Please help or tell me when normal service will be restored - thanks.

[#4]

Apologies for troubling anyone if the issue was at my end - I re-booted my computer and email is working again.

[#5]

These things happen ... and it can be difficult to offer appropriate advice when an issue so far off the wall and is not reproducible!

[#6]

Now the exact same thing is happening again this morning.  No email access, same error messages.  Computer reboot hasn't fixed it so yesterday's apparent success was a red herring.

[#7]

Apologies again, this appears to be my employers' (a large UK University) IT security arrangements identifying https://webmail.plus.net/ as a potentially malicious MITM risk.  Might be worth looking at.