This question has been answered by multiple community members. See the first response.
I upgraded my server to PHP7 last month but didn't encounter these issues with any of the MODX site.
Sounds like the root user might have taken ownership/permissions of everything when updating
namei -mo /pathtomodxsite/
Not sure if that can be done in cPanel but if you have console access then try:
namei -mo /pathtomodxsite/
That'll be one in the same, core/cache is the folder that MODX uses.