Hosting support solved it. Apparently I had some incompatible code in .htcaccess file and deleting it worked:
DELETE this code:
<IfModule mod_headers.c>
Header set Connection keep-alive
# Cache-control headers
# 2 timmar
Header set Cache-Control "max-age=7200, must-revalidate"
# 2 veckor
<filesMatch "\.(ico|pdf|flv|jpg|jpeg|png|gif|js|swf|css|less)$">
Header set Cache-Control "max-age=1209600, public"
</filesMatch>
<filesMatch "\.(xml|txt|html|htm)$">
Header set Cache-Control "max-age=1209600, private, must-revalidate"
</filesMatch>
</IfModule>