Meeting series disappeared in organizers calender

We have the same issue on different customers. For every customer there is a subset of about 15% of the users affected. Only Workaround for us was to work in Online Mode. The issues we are seeing are not only Meetings but also simple Calendar Appointments. We have created 6 Appointments the same day. 2 of them get synched to the mailbox and are visible on mobile phone and outook on the web. The remaining 4 appointments disappear and ends up in the local Failures Folder of Synchronization issues. The same also we saw, when users move a mail from their personal inbox to a shared mailbox. The email disappears from the in box, never shows up in the shared mailbox, and again the mail can be found in Local Failures. Next issue. when we Delete mails in the inbox, sometimes they still show up in outlook on the web and the mobile phone as unread. Basically outlook Client does not sync with Mailbox. We discovered also unplugging and replugging the network cable solves the issue for maybe a few hours to 2 days. we tried to create new OST File, new Outlook Profile, New Windows Profile several times, all with no effect.  We also tried to disable antivirus software, disable outlook Add-ins, no changes. Also after a reboot the issue persist. Anyone else experiencing this Issues in Outlook Desktop app with chache mode?