I got a better reply from our provider. They cannot enable AllowOverrides All on our server for security reason.
@KevinTheJedi Can you tell me which detailed functions osTicket needs?
At the moment this is configured:
Options -ExecCGI -FollowSymlinks -Includes +IncludesNoExec -Indexes -Multiviews +SymLinksIfOwnerMatch
AllowOverride AuthConfig FileInfo Indexes Limit Options=ExecCGI,Includes,IncludesNOEXEC,Indexes,MultiViews,SymLinksIfOwnerMatch