Josh Gad is steaming mad at Germany’s flagship airline … he says his international travel experience on Lufthansa has been a complete nightmare … and he’ll never book them again.

The “Frozen” star is going nuclear on Lufthansa on social media, accusing the airline of sticking him back in coach seats despite paying for business class on a flight from Frankfurt to Greece … a demotion Josh says is “insane.”

It only gets worse from there … Josh says when he landed in Athens, he had an email from Lufthansa informing him his luggage wasn’t on the flight. Josh says the airline told him there was simply too much baggage on the plane to load his gear.

What’s more, Josh says the worst thing may have been Lufthansa’s customer service … or apparent lack thereof.

When he was checking in for his flight out of Frankfurt, Josh says airline reps were basically shrugging at folks who were begging for help after missing their flights … claiming the response was basically, “Yeah, sorry, too bad.”

It sounds pretty bad … and it must have been for Josh to speak out … he says he’s “the chillest traveler and I always give everyone in the service industry a pass because it’s truly hard but what I saw today is beyond the pale.”