I have also the problem, that OSSN is not working correctly with PHP > 7.0.
I have installed PHP 7.2 on my NAS, when I start OSSN after installing new components or themes or even when I'm logging into my OSSN, i get the error message:
SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data
I had to close browser and after that, it mostly works. Not always