Can't login "The submitted form was invalid. Try submitting again."

[Leonick]

Support Request Template
What version of phpBB are you using? phpBB 3.3.11
What is your board's URL? https://kestrel-minority.com/forum/
Who do you host your board with? Linode
How did you install your board? I used the download package from phpBB.com
What is the most recent action performed on your board? Update from a previous version of phpBB3
Is registration required to reproduce this issue? No
Do you have any MODs installed? No
Do you have any extensions installed? No
What version of phpBB3 did you update from? phpBB 3.2.9
What styles do you currently have installed? Prosilver
What language(s) is your board currently using? English
Which database type/version are you using? MariaDB
What is your level of experience? Comfortable with PHP and phpBB
What actions did you take (updating your board; installing a MOD, style or extension; etc.) prior to this problem becoming noticeable? Problem actually occurred a while ago, but haven't bothered looking in to in until now so a bit uncertain on some details.
Fairly sure I was on phpBB 3.2.x. After updating the server os I ended up with a php version that didn't work with the forum. I updated to 3.3.10 and it worked at least initially but has since broken.

Today I've updated to 3.3.11 using advanced updated. I've also made a full package update in /forum2. Same login issue in both.
Please describe your problem. Can't login, get error "The submitted form was invalid. Try submitting again."

Registration seem broken too, care less about this but worth mentioning.
Before you got to the form but couldn't submit, was a message almost of screen in bottom right about invisible captcha so tried disabling extensions and clearing cache, since then it's just asking to accept terms over and over, even with extensions enabled again.
Generated by SRT Generator

[Mick]

Is registration required to reproduce this issue? No

Can you supply a regular user account to test please?

Did you try using a different browser and or purging the browser cache?

I see you have Cloudflare enabled, have you tried disabling that or, at least, clearing the Cloudflare cache?

[Leonick]

Seems like a cloudflare issue...

I've tried a different device and incognito mode. I did clear the cache in Cloudflare to no effect. When disabling Cloudflare my browser didn't let me go to site at all because of HSTS.

Tried now again editing out ssl completely from the config and a different browser and with that login works (beside redirecting to https on login).
Guess I'll ditch Cloudflare unless anyone has any idea what the problem might be there?

[nomadic one]

I experienced this same error today, too. I was logged in just fine on my mobile device with Safari, but on my laptop, I was unable to log in with Firefox browser.

I referenced this topic. They suggested an issue with cookies, but that a regular cache clearing didn't resolve their issue. My issue was fixed after I closed my web browser and started a new session, without performing additional steps though. I don't think I have Cloudflare on my board, if that's relevant.