"Connect to the server to view them" error issues with personal and shared mailboxes in Outlook client ( Office 365 - Windows 10)

Hi everyone!

I'm working in a fairly large environment providing M365 support for users and have been seeing a lot of support tickets regarding this error message. We started by migrating users personal mailboxes to Exchange and then about a month later we migrated shared mailboxes to Exchange as well. Users have been given the proper permissions and have been configuring these shared mailboxes in their Outlook. We really didn't start noticing these support tickets until after the shared mailboxes had been migrated and users were starting to configure them.

I met with one particular user and looked through their personal mailbox and then looked through a few of their shared mailboxes. This user told me that they first started noticing this error message in their Outlook client ever since they configured their shared mailboxes in the client (over the past week). This error is "There are more items in this folder on the server - Connect to the sever to view them" but the user is definitely connected to the Exchange server. This user told me that it's been a major hit and miss with this error but when they first noticed the error it appeared in both the personal and shared mailbox folders/sub-folders and that both of these mailboxes would either be working fine (no error, can click the link to get older messages) or they wouldn't be working (error is present, can't get older messages).

When I met with the user though this changed, their personal mailbox was working fine but all of the shared mailboxes were showing this error. The really strange thing is that when I clicked into the first sub-folder in one of these shared mailboxes it showed the "There are more items in this folder on the server - Click here to view more on Microsoft Exchange" link which it should being the users cache settings are set to 1 year for each mailbox in their client and they have a lot of emails that are 1+ years old, but then it flickered between the link and the error about 10 times in 3 seconds and then landed on the error message. After that, each sub-folder I clicked into within all shared mailboxes it didn't glitch again like before but it just showed the error message.

I found an article from several years ago regarding this error where someone recommended to increase the Outlook client cache settings for each mailbox to "Full" and we're looking into testing this and then possibly implementing a global policy that will increase the max OST size limit and then will set the cache settings to "Full" for each user but we're really trying to figure out the actual cause behind this to better understand what's going on. I find it to be a huge coincidence that this all started happening right after the shared mailboxes were being configured by the users in their Outlook but I don't know why that would effect anything.

Does anyone have any ideas as to what could be causing this error, and even if anyone can suggest anything else to try for a potential fix that would be great as well! Thanks! So far, we've been suggesting that users utilize OWA so that they don't have to worry about getting this error but a lot of users need to use several shared mailboxes and they don't like the feel of it because they have to manually open each shared MB in a new tab.

Working link:

Error message (no working link):