I have the exact same problem. 
Als Antwort auf Ramon Figueroa
Re: NEW SameSite=None; Secure Cookie Settings breaks LTI
von Veronica Volz -
This has been fixed. You will need to update to Moodle 3.8.1+.
Als Antwort auf Veronica Volz
Re: NEW SameSite=None; Secure Cookie Settings breaks LTI
von Randy Thornton -
This fix has also been backported to 3.5.11, 3.6.9, 3.7.5 as well, per MDL-67175.