Changed router to Plusnet model, now getting Outlook warning re site (cloudflare) security cert fail

[#1]

Hi - changed my router back from Draytek to the provided Plusnet Hub Two due to OpenReach line issues (to simplify their testing of line).

Noticed that am now getting intermittent error messages related to outlook.office365.com:

'Security Alert ... outlook.office365.com... problem with the site's security certificate...the name on the security certificate is invalid or does not match the name of the site" and options are Yes/No/View Cert. Looking at the cert it was issued to cloudflare-dns.com.

Any ideas? the Plusnet router shows the usual 212.x.x.x stuff for DNS, though I also note that the Firewall defaults to On.

Thanks.

(incidentally, despite 48 down / 10 up speed showing, web pages on my PC taking ages to load - tracert to 212.x.x.x.x 's timing out and to 8.8.8.8 taking ages).

[#2]

Does this describe your issue - "The name on the security certificate is invalid or does not match the name of the site" error in Ou...

I would be surprised if this is a router caused problem - there is some (remote?) possibility that this is related a DNS look up issue with the auto-discovery service.

[#3]

Hi - thanks for your reply.

No, it turns out that it is not just Outlook which is affected. I tried to open a cloud-stored PDF file from within Acrobat and got pretty much the same message, again indicating Cloudflare.

The bizarre thing is that this is strangely intermittent - I can go the whole day with no problems then suddenly e.g. Outlook will keep popping up the Cert message.