Call to undefined function simplexml_load_file()

After my host upgraded to php 7 I now get the following error when trying to login:

Call to undefined function simplexml_load_file()

There is a thread on wordpress.org about that problem (https://wordpress.org/support/topic/simplexml_load_file-undefined) however, in my php.ini there is written in capitals (presumably from LiveConfig): "DO NOT MODIFY. ALL CHANGES WILL BE OVERWRITTEN". - so what can I do (not activate support access, obviously - as I don't have access to my backend :wink: )? Any ideas?