1) The reporting of that kind of error is not a good idea to switch off – what do you think the code can do if a plugin it calls is not there ?
2) No such problems are being experienced locally or on the test site. https://test.icalevents.com/
Without further information (hint – the actual message may have been helpful) one has to assume that the problem is specific to that install. Again if one had the function name one could identify whether it was a version problem (eg missing wp or php function) or a mismatched or corrupted plugin code.
It sounds like
1) you installed or upgraded a plugin without putting the live site into maintenance mode and without testing. I suggest at least you use a plugin like https://www.ads-software.com/extend/plugins/maintenance-mode/ when doing updates.
2) infrequently code sometimes does not get uploaded with integrity – ie there is a missing or corrupted file. Try deleting the plugin, then reloading the plugin.
