Your software
My Mautic version is: just updated
My PHP version is: 7.2
Your problem
My problem is: 431 errors and unavailable screens
These errors are showing in the log:
Steps I have tried to fix the problem:
Much testing.
First guess was long header, but looking in the debugger said otherwise.
Unfortunately, shortening my quite long naming conventions fixed it for most but not all pages.
After a day or two it was back.
Switching browser fixed it.
Then it was back
I cleared wordpress cache and disabled wp_security in case they were impacting because mautic is in a sub directory of wordpress. no help
Hired an expert, that lead to an update which brought its own unique problems then back to the same old grind.
After a day away form it, I realised it had to be cookie sizes.
I checked and sure enough there are a lot of huge cookies set by mautic.
I researched and sure enough many browsers lead by chrome have dramatically reduced the max storage of cookies in recent versions.
I deleted all cookies for mautic on all browsers and everything worked just fine.
Why is everyone not screaming about this?
I was testing continuously:
43 emails and 88 landing pages; that’s a lot of clicking and testing and cookies.
Of course I could be wrong about all of it. Maybe I am having a golden patch this week.
You decide.
If Im right, then your solution will soon be only usable for amateurs with little websites