I think I have some more info which may be helpful:
Open a browser and in one tab, login and open a session to openemr.
Open a new tab, login and open a session to openemr. Works great.
First tab is no longer active with its own session.
Start over. We are going to open 3 tabs in the same browser.
Open a TAB 1, login and open a session to openemr.
Open a patient chart and get to work…
Wait, you have to answer a patient call.
To not destroy your work, you open a new tab TAB 2. You have to login again to the system and open another session to openemr. Open the chart of the patient on the phone and get to work. Great. Return to original tab. It looks like you are in Patient 1 chart. You are not. Click on Patient 1 to bring up their Dashboard and encounter page, and you get the “Site ID is missing from session data!”. By opening tab 2 and creating another session, you have just destroyed the session in TAB 1.
One a third tab TAB 3, login to openemr. Do nothing except switch back to TAB 2 and watch for changes… If you do nothing, you will still get this message, except it shows up in the New Messages indicator. If you do anything, TAB 2 will revert to “Site ID is missing from session data!”.