We will have to do an update to powerstore for php7. The mySQL version wouldn't matter if the settings were the same. You really just had to change that setting, but rolling back to a version with a different setting of course would work as well.
The important part of the error is:
Security errorSecurity header is not valid
This means your username, password, or security key is incorrect. Double check them with your paypal profile and cut and paste to reset them and it should solve your problem.