I have been going crazy over this issue. And so far i yet to hear back from whmcs support (been like almost 2 days).
I have set the secretword, i have white listed the 2checkout ips also in cfs whm/cpanel firewall.
I can get to the file tco.php when type it in the address bar. I have updated the api passowrd, setup secret words.
I have also uploaded an old version of tco but i keep getting that error in the gateway log.
The only reason i can think of this is happening is because of the file, maybe its encryption got corrupted or something like that.
After searching every where to solve the problem i am asking your help with the issue.
Hope some one can give me their file or tell me how to fix that or where to look on the server that might be the cause of this, or anything else.
thanks