We know we have to execute the ALLES() function. But the input is converted to lowercase.
Executing the function is done by assigning eval("alles()") to the existing 'a' variable. To get the string to uppercase, we can use`eval('a'.title()+'l'.title()+'l'.title()+'e'.title()+'s'.title()+'()')`To verify this, it can be wrapped in a print() or 'a' can be printed after the eval has been run.
Sending any data as a parameter results in a string being returned that is directly related to the input. Going through all the available options (~30 char strings of a-z and 0-9 excluding banned chars) we see that with the string of 3's gets us what looks like a flag.
Tweaking the string a bit until we get the expected ALLES{ prefix, we see a pattern developing. Continuing this 133713371337 pattern for the string gets us the complete flag.