custom coded forms don't function landing page not found

I want to set up a newsletter signup on one page of the website.
Therefore I first embedded the generated form with the shortcode [newsletter]. This worked really well, but the input fields have the same color as the background.
So I tried a Custom Coded Form. I just copied the generated code and inserted it in Form1.

Then I embedded it with the shortcode [newsletter form=1] and wanted to try out, if the form functions. The page for signup was built up correctly, but after trying to submit the landing page wasn't found. (A white page with "Invalid" written on it was shown.) So the url written down in the action-method is the url of my website.

After running it in debug mode I got this in the logfile:

[24-Apr-2020 19:15:28 UTC] PHP Fatal error:  Uncaught Error: Call to undefined method PHPCompatibility\Sniffs\Operators\RemovedTernaryAssociativitySniff::isShortTernary() in /mnt/web505/e0/82/54407382/htdocs/WordPress_01/wp-content/plugins/php-compatibility-checker/vendor/phpcompatibility/php-compatibility/PHPCompatibility/Sniffs/Operators/RemovedTernaryAssociativitySniff.php:125
Stack trace:
#0 /mnt/web505/e0/82/54407382/htdocs/WordPress_01/wp-content/plugins/php-compatibility-checker/vendor/squizlabs/php_codesniffer/CodeSniffer/File.php(576): PHPCompatibility\Sniffs\Operators\RemovedTernaryAssociativitySniff->process(Object(PHP_CodeSniffer_File), 3659)
#1 /mnt/web505/e0/82/54407382/htdocs/WordPress_01/wp-content/plugins/php-compatibility-checker/vendor/squizlabs/php_codesniffer/CodeSniffer.php(1894): PHP_CodeSniffer_File->start(NULL)
#2 /mnt/web505/e0/82/54407382/htdocs/WordPress_01/wp-content/plugins/php-compatibility-checker/vendor/squizlabs/php_codesniffer/CodeSniffer.php(1772): PHP_CodeSniffer->_processFile('/mnt/web505/e0/...', NULL)
#3 /mnt/web5 in /mnt/web505/e0/82/54407382/htdocs/WordPress_01/wp-content/plugins/php-compatibility-checker/vendor/phpcompatibility/php-compatibility/PHPCompatibility/Sniffs/Operators/RemovedTernaryAssociativitySniff.php on line 125

So I just don't know what to do to make the custom forms run.
Maybe you can help

Thanks.