@affa85 My propose:
I do below instruction - and gmail works fine 3days wihout errors.
i compere cert from operaGX with cert from another browser - for safety when i have same.
On secend step i save cert (with i have problem) to disk
Click to Padlock icon (left from URL) -> Click on Certyfication -> on new windows change tab to detailes -> Copy to files... -> save to disk
Third step - import cert in Opera options Security tab -> manage certificates -> to:
"intermediate certification authorities" and
"trusted root certification authorities"
(names of tab i got from google translator - i have PL version of Opera)