Thank you, the Pages error seems to not just be related to templates as it is happening also in the ACP so we will need to investigate that further.
As the error message is stating an unknown function, just to ensure there are no missing or out of date files that the system isn't detecting, could you please go to the Client Area and download a full set of files then extract these on your local computer. Once done, upload the "ips_****" folder to your server, overwriting what is there. Once this has completed, please check again.
Regarding the Theme issue, do you have mod_security or any other security module enabled on your server? I'd like to ensure that is not getting blocked by it. Please have your hosting provide disable the module or add an exception for any errors in the log.