It seems that the host has Imagick installed which is great since it is faster and uses less resources than the default GD library for working with images but unfortunately they have a pretty old version installed which does not have the setImageOpacity().
A temporary fix would be to disable/comment out that function.
Open the file mention in the error message:
Go to line 38.
Put two forward slashes in the very front of the line so it looks like this:
//$this->image->setImageOpacity( $level );
Save the file and overwrite the original.
This will not affect the functionality of Connections so it is a safe edit. Version 8.1.6 will contain a fix for this so it’ll be safe to update to it when I release it, soon’ish.
Apologies for the trouble!