when i try to access a shared mailbox the logfile shows me this and access isn't possible

Jun 15 19:38:57 imap(michael@bigmichi1.de): Error: mdbox
/srv/vmail/mail/bigmichi1.de/familie/mailboxes/INBOX/dbox-Mails:
Invalid dbox header size: 0 Jun 15 19:38:57 imap(michael@bigmichi1.de): Warning: fscking index
file
/srv/vmail/indexes/shared/bigmichi1.de/michael/shared/familie@bigmichi1.de/storage/dovecot.map.index Jun 15 19:38:57 imap(michael@bigmichi1.de): Warning: mdbox
/srv/vmail/mail/bigmichi1.de/familie/storage: rebuilding indexes

doveconf -n

# 2.2.2 (30f00db1a8b0): /etc/dovecot/dovecot.conf # OS: Linux 3.8.0-25-generic x86_64 Ubuntu 13.04 ext4 auth_mechanisms = plain login debug_log_path = /var/log/dovecot/debug.log dict { acl = pgsql:/etc/dovecot/dovecot-dict-sql.conf.ext expire = pgsql:/etc/dovecot/dovecot-dict-sql.conf.ext quota = pgsql:/etc/dovecot/dovecot-dict-sql.conf.ext } first_valid_gid = 8 first_valid_uid = 8 info_log_path = /var/log/dovecot/info.log last_valid_gid = 8 last_valid_uid = 8 log_path = /var/log/dovecot/error.log mail_debug = yes mail_gid = 8 mail_location =
mdbox:/srv/vmail/mail/%d/%n:INDEXPVT=/srv/vmail/indexes/private/%d/%n:INDEX=/srv/vmail/indexes/shared/%d/%n mail_plugins = quota zlib acl expire virtual mail_uid = 8 mailbox_list_index = yes managesieve_notify_capability = mailto managesieve_sieve_capability = fileinto reject envelope
encoded-character vacation subaddress comparator-i;ascii-numeric
relational regex imap4flags copy include variab les body enotify environment mailbox date ihave imapflags notify mdbox_preallocate_space = yes mdbox_rotate_size = 10 M namespace { list = children location =
mdbox:/srv/vmail/mail/%%d/%%n:INDEXPVT=/srv/vmail/indexes/private/%d/%n/shared/%%u:INDEX=/srv/vmail/indexes/shared/%d/%n/shared/%%u prefix = Shared/%%u/ separator = / subscriptions = no type = shared } namespace inbox { inbox = yes location = mailbox Archive { auto = subscribe special_use = \Archive } mailbox Drafts { auto = subscribe special_use = \Drafts } mailbox Sent { auto = subscribe special_use = \Sent } mailbox Spam { auto = subscribe special_use = \Junk } mailbox Trash { auto = subscribe special_use = \Trash } mailbox virtual/All { auto = subscribe special_use = \All } mailbox virtual/Flagged { auto = subscribe special_use = \Flagged } prefix = separator = / type = private } namespace virtual { location =
virtual:/srv/vmail/virtual:INDEXPVT=/srv/vmail/indexes/private/%d/%n/virtual:INDEX=/srv/vmail/indexes/shared/%d/%n/virtual prefix = virtual/ separator = / } passdb { args = /etc/dovecot/dovecot-sql.conf.ext driver = sql } plugin { acl = vfile:/srv/vmail/acl:cache_secs=300 acl_anyone = allow acl_shared_dict = proxy::acl expire = Trash 7 Trash/* 7 Spam 30 expire_dict = proxy::expire quota = dict:User quota::proxy::quota quota_warning = storage=95%% quota-warning 95 %u quota_warning2 = storage=80%% quota-warning 80 %u quota_warning3 = -storage=100%% quota-warning below %u sieve = /srv/vmail/sieve/%d/%n/.dovecot.sieve sieve_default = /srv/vmail/sieve/default.sieve sieve_dir = /srv/vmail/sieve/%d/%n/ sieve_extensions = +notify +imapflags sieve_global_dir = /srv/vmail/sieve/global } postmaster_address = postmaster@bigmichi1.de protocols = " imap sieve" service auth-worker { user = $default_internal_user } service auth { unix_listener /var/spool/postfix/private/auth { group = postfix mode = 0666 user = postfix } unix_listener auth-userdb { group = mail mode = 0666 user = mail } } service dict { unix_listener dict { group = mail mode = 0660 user = mail } } service managesieve-login { inet_listener sieve { port = 4190 } } service quota-warning { executable = script /usr/local/bin/quota-warning.sh unix_listener quota-warning { user = mail } user = dovecot } ssl_cert =