Hi Josh,
I would like to confirm the following information with you:
1.What's the service pack version of your Hybrid Exchange 2010 Server? Please update it to SP3 to test this issue.
2.Can you move the problematic mailbox to your on-premises exchange server via EAC? Please refer to this article for detailed steps.
3.When you mentioned that you have removed the contact list, where did you remove the contact list? Please also check in Outlook client.
In addition, I thought this issue may be related the new feature "Unified Contact Store" introduced in Lync Server 2013. But I didn't find any information about how to disable this feature for Lync Online users. I will do some further research and consult some senior engineers with this issue, if there are any updates, I will post here. Thank you for your understanding.
Thanks,
Jolin Qiao
Jolin Qiao