there's an error about language files. I switched the whole phpbb installation to english, but it won't help:
[Sat Feb 06 15:17:32.997940 2021] [lsapi:error] [pid 62999:tid 140129900902144] [client xxx] [host
www.xxx.xx] Backend fatal error: PHP Fatal error: Uncaught phpbb\\language\\exception\\language_file_not_found: Language file ./../language/en/acp_tools_mailtopost.php couldn't be opened. in /home/xxx/domains/xxx.xx/public_html/forum/phpbb/language/language_file_loader.php:186\nStack trace:\n#0 /home/xxx/domains/xxx.xx/public_html/forum/phpbb/language/language_file_loader.php(143): phpbb\\language\\language_file_loader->get_language_file_path('./../language/', 'acp_tools_mailt...', Array)\n#1 /home/xxx/domains/xxx.xx/public_html/forum/phpbb/language/language_file_loader.php(77): phpbb\\language\\language_file_loader->load_file('./../language/', 'acp_tools_mailt...', Array, Array)\n#2 /home/xxx/domains/xxx.xx/public_html/forum/phpbb/language/language.php(625): phpbb\\language\\language_file_loader->load('acp_tools_mailt...', Array, Array)\n#3 /home/xxx/domains/xxx.xx/public_html/forum/phpbb/language/language.php(184): phpbb\\language\\language->load_core_file('acp_tools_mailt...')\n#4 /home/xxx/domains/xxx in /home/xxx/domains/xxx.xx/public_html/forum/phpbb/language/language_file_loader.php on line 186\n, referer:
https://www.xxx.xx/forum/adm/index.php? ... bc28f&i=32