Did you manage to upgrade those plugins you mentioned earlier as well?
Might be a good time to ask 'customer' to review/research what level of service (plan) they have with provider. Moodle sites that are used only grow ... they don't get smaller ... thus more mem/resources + pretty soon here customer should be seeing notifications the version they are running is
Might run environment check and update component, then set to highest version of moodle (which is now 3.8+) to see if server continues to meet requirements. Uhhh ... the check doesn't address memory. That's before you get 'bushwhacked' again to do upgrade over a holiday! :|
Bug fixes for general core bugs in 3.5.x ended May 2019 (12 months). Bug fixes for security issues in 3.5.x will end 10 May 2021 (36 months).