MyBB Community Forums

As you supposed:

Yes. Problem wasn't there until I bought Domain and configured CF (Free SSL)

Can I login normally (outside of ACP)?

Yes, I can!

Can I login in ACP?

Yes, I can, but it logs me out after clicking anything with the mentioned error...

My .htaccess:

Options -MultiViews +FollowSymlinks -Indexes

#
# If mod_security is enabled, attempt to disable it.
# - Note, this will work on the majority of hosts but on
#   MediaTemple, it is known to cause random Internal Server
#   errors. For MediaTemple, please remove the block below
#
<IfModule mod_security.c>
	# Turn off mod_security filtering.
	SecFilterEngine Off

	# The below probably isn't needed, but better safe than sorry.
	SecFilterScanPOST Off
</IfModule>

#
# MyBB "search engine friendly" URL rewrites
# - Note, for these to work with MyBB please make sure you have
#   the setting enabled in the Admin CP and you have this file
#   named .htaccess
#
<IfModule mod_rewrite.c>
	RewriteEngine on
	RewriteCond %{HTTPS} off
	RewriteCond %{HTTP:X-Forwarded-Proto} !https
	RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
	RewriteRule ^forum-([0-9]+)-page-([0-9]+)\.html$ forumdisplay.php?fid=$1&page=$2 [L,QSA]

	RewriteRule ^thread-([0-9]+)\.html$ showthread.php?tid=$1 [L,QSA]
	RewriteRule ^thread-([0-9]+)-page-([0-9]+)\.html$ showthread.php?tid=$1&page=$2 [L,QSA]
	RewriteRule ^thread-([0-9]+)-lastpost\.html$ showthread.php?tid=$1&action=lastpost [L,QSA]
	RewriteRule ^thread-([0-9]+)-nextnewest\.html$ showthread.php?tid=$1&action=nextnewest [L,QSA]
	RewriteRule ^thread-([0-9]+)-nextoldest\.html$ showthread.php?tid=$1&action=nextoldest [L,QSA]
	RewriteRule ^thread-([0-9]+)-newpost\.html$ showthread.php?tid=$1&action=newpost [L,QSA]
	RewriteRule ^thread-([0-9]+)-post-([0-9]+)\.html$ showthread.php?tid=$1&pid=$2 [L,QSA]

	RewriteRule ^post-([0-9]+)\.html$ showthread.php?pid=$1 [L,QSA]

	RewriteRule ^announcement-([0-9]+)\.html$ announcements.php?aid=$1 [L,QSA]

	RewriteRule ^user-([0-9]+)\.html$ member.php?action=profile&uid=$1 [L,QSA]

	RewriteRule ^calendar-([0-9]+)\.html$ calendar.php?calendar=$1 [L,QSA]
	RewriteRule ^calendar-([0-9]+)-year-([0-9]+)-month-([0-9]+)\.html$ calendar.php?calendar=$1&year=$2&month=$3 [L,QSA]
	RewriteRule ^calendar-([0-9]+)-year-([0-9]+)-month-([0-9]+)-day-([0-9]+)\.html$ calendar.php?action=dayview&calendar=$1&year=$2&month=$3&day=$4 [L,QSA]
	RewriteRule ^calendar-([0-9]+)-week-(n?[0-9]+)\.html$ calendar.php?action=weekview&calendar=$1&week=$2 [L,QSA]

	RewriteRule ^event-([0-9]+)\.html$ calendar.php?action=event&eid=$1 [L,QSA]

	<IfModule mod_env.c>
		SetEnv SEO_SUPPORT 1
	</IfModule>
</IfModule>

#
# If Apache is compiled with built in mod_deflade/GZIP support
# then GZIP Javascript, CSS, HTML and XML so they're sent to
# the client faster.
#
<IfModule mod_deflate.c>
	AddOutputFilterByType DEFLATE text/css text/html application/xhtml+xml text/xml application/xml text/plain text/x-component application/javascript application/x-javascript application/rss+xml application/atom+xml application/json application/manifest+json application/x-web-app-manifest+json application/vnd.ms-fontobject application/font-sfnt application/font-woff application/font-woff2 image/svg+xml image/x-icon
</IfModule>

# Note: You are able to choose a different name in the Admin CP. If you've done that you need to change it here too
<Files "error.log">
	Order Deny,Allow
	Deny from all
</Files>

Thanks in advance!

---

Followed: https://community.mybb.com/thread-217367.html

& fixed via Cookie Deletion (I thought I already did that, lmao..)