Using EWS on Office 365 ends in 'ErrorTooManyObjectsOpened'

Hello,

we have a problem with our sync-tool which is using the EWS to sync appointments between our calender and the exchange calender. One "service" account is in use to access the calender of users which will be synchronized and this is not done via impersonation. This works fine on the most customer setups, regardless if it is a local exchange system or a Office 365 account. 

Now we have one customer that get's an error while loading properties for items. The error message is 'ErrorTooManyObjectsOpened'. So but this does not happen to all of the 20 users. Just a few, around 20 % get this error. Also if we temporary disable all users expect one which gets the error. 
What makes me also very "confuse" is, that we can use FindItems/FindAppointments without errors and the following LoadPropertiesForItemsget's the error. 

Dim nPropertySet As New PropertySet(BasePropertySet.FirstClassProperties, nAdditionalProperties)
nPropertySet.BasePropertySet = BasePropertySet.FirstClassProperties

FindResult = Service.FindAppointments(New FolderId(WellKnownFolderName.Calendar, New Mailbox(MyUser.User.Email)), nCalenderView)

For Each ObjAppointment As Appointment In nFindResult.Items
    ItemList.Add(ObjAppointment)
Next

Service.LoadPropertiesForItems(ItemList, nPropertySet)

We are using the EWS Managed API to access the EWS.

I found another post with this error, but the solutions does not work for us. We have done the "break" between the calls with a bigger resolution of 120sek every 5 users, but it does not work.

So it looks like there is another limitation that I don't know yet.