Actually the variable is used as the URL for HTTP-Requests. I’ve just printed it out using BP because only there you could see that just the first few characters are broken. The invalid-url error thrown by the HTTP-Request just prints out the wrong characters.
I found a workaround by defining the default value as a string literal inside the URL-Creation function and using the variable as an optional overwrite like this:
FString BaseURL;
if(MyClass.Server == "")
BaseURL = TEXT("https://somedomain.tld/something");
else
BaseURL = MyClass.Server;
I don’t know exactly why, but this works.