I don't see a screenshot of the certificate error.

Here it is. Thanks. I also get a message saying "certificate error for [my emailaddress]. Check your server settings". My server settings are correct.

That screen shot if from a phone, is this a question about Thunderbird for Android?

while I do not use the android product I will take a punt that the issue is you have an antivirus product installed on the phone and it is using it's own self signed encryption certificated to facilitate access to your encrypted communication for its scanning. But that is just a guess based on what the majority of certificate error on windows desktop machines is caused by.

I would also punt on the advanced button opening further details of exactly what the invalid certificate is all about. AS I do not have an antivirus scanning email, nor use the Android product I can do little more than guess.