Can't connect to Gmail on stock email app using WiFi (works on mobile data) This is on an Xperia X Compact with Android 7.0. I get the error message "Can't establish server connection"1 when I want to sync my email while connected to my home WiFi. 


Other IMAP accounts sync without a problem
Using the stock email app for Gmail worked fine on Marshmallow
Using Gmail in the stock email app works when I'm on mobile data (!)
Using other email apps (e.g. Gmail, K-9, BlueMail) to connect to Gmail while on WiFi works as well
I have been able to add the Gmail account and initially synchronize my emails on the same WiFi network without any problems
There is no error message under Accounts -> Google -> Synchronize Gmail


Things I have tried:


Deactivate and reactivate mail app   
Clear data of the mail app
Remove and re-add Google account
Complete factory reset via Xperia Companion
Disable two-factor auth
Add Gmail manually as an IMAP account in the mail app
Allowed less secure apps in Gmail settings
Check Gmail's IMAP settings on Desktop (it's enabled)


I haven't had the opportunity to test it on a different network than my home WiFi yet, but to me this looks more like a software problem since all other email clients, as well as the initial setup worked fine? There are of course a number of obvious workarounds (use Gmail / other email client), but I would prefer the stock email app to work as it fits my needs best. 



1 All error messages and descriptions of settings have been translated from German. Please edit if you know the exact English translations so other people with the same problem can find this question more easily.
As usual, I found the culprit minutes after posting this question when I battled with this problem for a week:

I use DNS66 for ad-blocking. Adding the email app to the exceptions and restarting the service solved the problem.