Forum MyBB, ERRORE 500

Ciao a tutti, è da ieri che all'improvviso non posso più accedere al mio forum perchè mi esce un "errore 500".
E' probabile che sia dovuto al file htaccess.txt
Volevo ricordare che prima di questo problema io avevo provato ad installare il SEO e anche uno script per vedere quante visite mensili c'erano.
Il forum si trova in: http://miosito.altervista.org/Upload
Vi posto anche il codice del file htaccess.txt

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
	RewriteRule ^forum-([0-9]+)\.html$ forumdisplay.php?fid=$1 [L,QSA]
	RewriteRule ^forum-([0-9]+)-page-([0-9]+)\.html$ 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=$1 [L,QSA]
	RewriteRule ^calendar-([0-9]+)-year-([0-9]+)\.html$ calendar.php?action=yearview&calendar=$1&year=$2 [L,QSA]
	RewriteRule ^calendar-([0-9]+)-year-([0-9]+)-month-([0-9]+)\.html$ 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 application/x-javascript text/css text/html text/xml
</IfModule>

Sapete come posso risolvere??? aiutatemi per favore

[Alessandro1]

Se nella root hai htaccess.txt e non .htaccess penso proprio che il problema non sia dovuto da quello che c'è dentro...è qualche altro file :)
Hai tolgo la cartella Upload quindi non possiamo guardare...

no tranquilli, ho risolto.
Ho messo l'htaccess.txt originale e l'ho rinominato in .htaccess