PHP 7.1 gateway 502

Dear,

We have an issue with our Wordpress website hosted by Forge -> Linode. Currently the host uses PHP 7.1 what seems O.K. in the past. This morning the websites throws an 502 gateway error.

What I tried so far is restarting nginx then the websites goes live, after reloading or visiting another page the website get the 502 error again.

I also created a new server with php 5.6 which is live now, so the error should be in the php version.

Is there anyway to downgrade the php version or another fix? Please help me as soon as possible. The website has allot of visitors everyday...