Q1: yes, your demo site is working flawlessly both from phone and web browser
Q2: No, no further changes at that time
Q3: Firefox ESR 24.6 on Linux
Q4: No changes to the menu code, but style.css changes on the menu
It is getting late here. Since the demo site is working, I will need to look into more details.
I have the header.php file copied into my child theme folder because there is a tiny modification in the head. But this is far away from any menu code, and everything else is identical to your file: I only have a condition (if !is_page == …) around the title tage because I don’t need this on a certain page.
What I am also wondering: can this be related to any caching on the server side or even locally?
My next steps (tomorrow):
a) Repeat update and test again
b) If Child Theme is broken, switch to Encounters and check function again
When Encounters itself works, I will need to dive deep into style.css and review the header.php change one more time.
Thanks for the quick help, I will come back with more details.