I think I might have found the source of the problem, and it’s actually a redirection within the transplosh plugin. I don’t fully understand of course, but there are several redirections, I guess to get to the correct translation page. This redirection however causes to empty the $_POST variables..
I have found a solution however, I have added a few lines of code to the search plugin to avoid this redirection if possible (by detecting the language and such), so the $_POST remains in tact. I won’t be able to make a permanent solution out of this unfortunately, so please do not update the plugin if you plan to use transplosh in the future, otherwise these changes will be lost.