2 Branches, 2 different Builds, both have problems

hello,
i am desperate now and after trying everything, I hope to find some help here.
please let me explain:

I have one git repository with 2 branches:

- php7
- php8

for both branches I created two different build setups:

php7

Default branch: refs/heads/php7
Branch specification:

+:refs/heads/(feature-*)
+:refs/heads/(WT-*)
-:refs/heads/php8

excluding php8 here is just a test of mine, I know it shouldn´t be needed.

Branch Filter:

+:refs/heads/WT-*
+:refs/heads/feature-*
+:<default>
-:refs/heads/php8

same here...

php8

Default branch: refs/heads/php8
Branch specification: <empty>
Branch Filter: +:<default>

I don´t use any triggers, teamcity is fetching every X seconds for changes.

here´s the problem:

when I push something to branch php8, the following happens:

- the build-setup for php8 does nothing, I only see a new entry in the "Pending Changes" Tabs, there I have to run the build manually so that the build finally gets started

- the build-setup for php7 gets an error with 

Failed to collect changes, error: Builds in php8 branch are disabled in build configuration

which is not totally wrong, but the problem is:

- I get a slack notification with that error every time I push something to php8
- I see an error in my php7 build list every time, but it shouldn´t listen at all to php8 pushes

so my goal is:

- php8 builds should start as it does with my php7 (and I don´t really understand why my php7 build works, and my php8 not, because I copied php8 from php7)

- php7 should not get triggered at all if anything happens inside the branch php8

my teamcity is not up to date, sorry: TeamCity 2021.2.3 (build 99711) 
but my feeling tells me, it´s not a bug, more a problem with my configuration.

thanks for any help!!