In Moodle code there are a lot of calls of this type:

get_context_instance(CONTEXT_SYSTEM, SITEID)

And, when requesting for CONTEXT_SYSTEM we don't need to specify SITEID any more.

So, all the sentences above should be changed to:

get_context_instance(CONTEXT_SYSTEM)

There are aprox. 130 changes to perform, distributed along aprox. 90 files. I would suggest to do the change progressively, fixing 3-4 files each day, both in 19_STABLE and HEAD.

Ciao :-)