Hello Rudy, thank you for replying; I will answer your questions below:
1 - Yes, it also happens in Chrome in the same way, it just gives me a slightly different message (Uncaught RuntimeError: memory access out of bounds).
2 - Yes it happens on a new clean HTML5/MOBILE project with no starter content, I wll post the steps below.
3/4 - Blank project - HTML5/MOBILE - High Quality - No starter content,
a. Create NewWidget BP - add 1 button with Tooltip text.
b. Create ToolTipWidget BP and add 1 Vertical Box - all default + “size to content” - Check “Is
variable”. (img1)
c. Add Text Block inside Vertical box - all default only Horizontal/vertical alignment tweaked - Check “Is
variable” (img1)
d. Create nodes inside ToolTip Widget Graph (img2)
e. Bind Button to ToolTipWidget BP (img3)
f. Level BP: Event begin play - Create NewWidget BP - Add to viewport
Test: PIE works, but launch on browser does not work (Chrome/Firefox)
Please note that with the blank project i get a slightly different message again but once I click OK it’s the same as before. (img4)
As usual, if I disable the ToolTip bind and use the default Tooltips it works fine. (img5)
Thank you for your time, hope we can find a solution 
Plesae let me know if I forgot something, or if you need more info.
Have a nice day
Giacomo