There can be hundreds of reasons for those warnings, php version change or settings changed. Previously many errors were silenced in osclass we have removed that behaviour from many places and will continue to do so.
This behaviour allowed crappy code to fail silently which is not a good thing and create hard to debug errors.
We have also upgraded many libraries in osclass which also throw new errors and exception if wrong arguments passed.
In last we have changed so many things in osclass core and it’s possible that those changes can break things and we are here to help in those cases.
But in this case that fatal error has nothing to do with osclass, the error was that “invalid email address passed” and when I checked plugin’s file I found an empty string ‘’ was passed as email address to phpMailer library and library rightly throwing the error. It should be checked in a plugin before passing it to phpMailer library. I added that check and fatal error got fixed.
Now it may seem that new osclass is making your installation unstable but in reality, it is uncovering the previously hidden issues.