Browser: chrome seems to have difficulty resolving caching sometimes. Firefox seems better.
PHP: with 7.2 everything works, both 7.3 and 7.4 didn’t work.
Firewall and hosting: I feel like some kind of punching bag. Hosting days, theme is the problem, theme says hosting is the problem … I need a lasting solution! And I understand hosting quite well, that the massive have problems with the theme architecture because of calls because of the API keys for updating. Could you maybe have a look again in debug mode, whether the errors messages still occur under 7.2?