Hi Andreas,
This a bug in zimbra UI when the acount has an externel POP/IMAP configured.
Work around : switch to the Standard UI to accept the share.
Work around 2 : replace external POP/IMAP with fetchmail.
Ref :
Bug 23582 – No Accept/Decline buttons on share request
Best regards,
Florent Manens
StarXpert : Expert en migration OpenOffice.org, Zimbra et Alfresco