Hey,
Firstly: Thank you for your feedback, but I have found the solution on my own. Even though your answer is not the cause of the problems, I think it might be a good idea to later switch to something like Redis for example!
In this case the problem was simple (and a bit stupid I guess). It turns out that running services that heavily depend on dates and schedules (like railways do in most cases) on a provider with a different timezone is not a good idea. After looking at the data I realized, due to Germany being GMT+1 my API was fetching data for the past hour and this caused the problems.
Sorry for bothering you as this is clearly not Vercel’s fault.
Have a nice day
crwntec