Changed SCRIPT_NAME to dovecot.sieve on Open Xchange´s mailfilter.properties, also port to 4190.
Activated mail_debug for Dovecot.
Login with Open Xchange and going to the filter settings bringt this in dovecot.log:
Jan 07 11:09:56 imap-login: Info: Login: user=<xxx@company.de>, method=PLAIN, rip=1.78.143.244, lip=1.78.143.246, TLS
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Loading modules from directory: /usr/lib/dovecot/modules/imap
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib01_acl_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib02_imap_acl_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib10_quota_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib11_imap_quota_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib20_autocreate_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Effective uid=1001, gid=1001, home=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25/
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota root: name=user backend=dict args=:proxy::quotadict
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota rule: root=user mailbox=* bytes=0 messages=0
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota warning: bytes=0 (85%) messages=0 command=/usr/local/bin/dovecot-quota-warning.sh 85
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota warning: bytes=0 (90%) messages=0 command=/usr/local/bin/dovecot-quota-warning.sh 90
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota warning: bytes=0 (95%) messages=0 command=/usr/local/bin/dovecot-quota-warning.sh 95
Jan 07 11:09:56 IMAP(xxx@company.de): Info: dict quota: user=xxx@company.de, uri=proxy::quotadict, noenforcing=0
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Namespace: type=private, prefix=, sep=/, inbox=yes, hidden=no, list=yes, subscriptions=yes
Jan 07 11:09:56 IMAP(xxx@company.de): Info: maildir: data=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir/
Jan 07 11:09:56 IMAP(xxx@company.de): Info: maildir++: root=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir, index=, control=, inbox=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl: initializing backend with data: vfile
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl: acl username = xxx@company.de
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl: owner = 1
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl vfile: Global ACL directory: (null)
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Namespace: type=shared, prefix=Shared/%u/, sep=/, inbox=no, hidden=no, list=children, subscriptions=yes
Jan 07 11:09:56 IMAP(xxx@company.de): Info: shared: root=, index=, control=, inbox=
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Namespace : Using permissions from /var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir: mode=0700 gid=-1
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl vfile: file /var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir/dovecot-acl not found
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Disconnected: Logged out bytes=60/910
Jan 07 11:09:56 imap-login: Info: Login: user=<xxx@company.de>, method=PLAIN, rip=1.78.143.244, lip=1.78.143.246, TLS
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Loading modules from directory: /usr/lib/dovecot/modules/imap
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib01_acl_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib02_imap_acl_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib10_quota_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib11_imap_quota_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Module loaded: /usr/lib/dovecot/modules/imap/lib20_autocreate_plugin.so
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Effective uid=1001, gid=1001, home=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25/
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota root: name=user backend=dict args=:proxy::quotadict
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota rule: root=user mailbox=* bytes=0 messages=0
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota warning: bytes=0 (85%) messages=0 command=/usr/local/bin/dovecot-quota-warning.sh 85
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota warning: bytes=0 (90%) messages=0 command=/usr/local/bin/dovecot-quota-warning.sh 90
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Quota warning: bytes=0 (95%) messages=0 command=/usr/local/bin/dovecot-quota-warning.sh 95
Jan 07 11:09:56 IMAP(xxx@company.de): Info: dict quota: user=xxx@company.de, uri=proxy::quotadict, noenforcing=0
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Namespace: type=private, prefix=, sep=/, inbox=yes, hidden=no, list=yes, subscriptions=yes
Jan 07 11:09:56 IMAP(xxx@company.de): Info: maildir: data=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir/
Jan 07 11:09:56 IMAP(xxx@company.de): Info: maildir++: root=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir, index=, control=, inbox=/var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl: initializing backend with data: vfile
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl: acl username = xxx@company.de
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl: owner = 1
Jan 07 11:09:56 IMAP(xxx@company.de): Info: acl vfile: Global ACL directory: (null)
Jan 07 11:09:56 IMAP(xxx@company.de): Info: Namespace: type=shared, prefix=Shared/%u/, sep=/, inbox=no, hidden=no, list=children, subscriptions=yes
Jan 07 11:09:56 IMAP(xx@company.de): Info: shared: root=, index=, control=, inbox=
Jan 07 11:09:56 IMAP(xx@company.de): Info: Namespace : Using permissions from /var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir: mode=0700 gid=-1
Jan 07 11:09:56 IMAP(xx@company.de): Info: acl vfile: file /var/vmail/vmail1/company.de/x/x/x/xxx-2011.11.14.21.37.25//Maildir/dovecot-acl not found