I cannot reproduce it on our forums so I will need little help on your side

OK - I checked source and it is caused by some plugin using hook "postbit_display_complete". vBET has 2 plugins using this hook. Can you please determine which one causes troubles on your forum? You can simply do it by disabling one and check does issue still exists. If yes then enable it and disable other one. If still exist then try with booth disabled. If it still exists - then most probably it is not vBET fault - try do disable whole vBET and check.

You will disable plugin by Admin CP-> Plugins & Products -> Plugin Manager
You will disable whole mod by Admin CP-> Plugins & Products -> Manage Products

Please let me know about results. If you PM me access details I will check it myself