I will "reopen" this post but, just today, I have this issue (The CSRF token is invalid. Pleas try to resubmit the form.) just after to update PS to 1.7.8.6, and after tried to downgrade PHP to 7.3 and 7.2 with the same issue:
Additional, the error This value should not be blank in a filled field...
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As I have checked and the issue happens only in a specific product, I will continue this issue here: