Well this is a hard one, some tests show it is due to server (The server encountered an internal error or misconfiguration and was unable to complete your request.
More information about this error may be available in the server error log.)
More shows it does not know what page (or header alike) has to be displayed.. Could very well be that server did change something, and php versions change can very well have effect. (also sanitizing eg security is called...some illegal posting?)
First option : do ONLY change back to php version.Test.
or enter site using http://lonestarpokertour.net/e107_admin/admin.php if you can find any things inside...
BACKUP ALL before you go to the next part..
(in a further case if things are getting resolved and looked into >, you might update the sites software with files from the Github branch as they are capable of running php 7 and on going version ( tests were + on 7.2.2) https://github.com/e107inc/e107. Please do not do this before you are sure what happened !
Please also read this https://github.com/e107inc/e107/issues/2152 if it comes near your issue..