Error 400

Dear support team,

after installing MailPoet 3 (Version 3.7.2 running on wordpress version 4.9.6) I am experiencing the following problem:

Trying to log in into the backend of WordPress using the Chrome browser (Version: 66.0.3359.181 (Official Build) (64-Bit)) leads to an error 400 with the following message:
“Bad Request
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.”

So I am unable to log in to my website using my standard browser. As a workaround I tried to log in using a different Browser (Mozilla Firefox Quantum Version: 60.0.1 (64-Bit) and this works just fine.
Could you please take a look at this issue?
Thanks!

The page I need help with: [log in to see the link]