Well you didn't mention what Exchange patch level but what you describe is "known" in Sp2 RU4 and RU5 and Microsoft mention "might" RU3.
However for us it didn't happen before RU4.
The issue is you get timeout from Exchange due Exchange is throttling.
Blackberry have an article KB33238 "Responsecode:ErrorServerbusy."
The workaround until a permanent fix is either disable EWS on the BES or increase the EWSFindCountLimit in the default throttlepolicy on Exchange.
btsc.webapps.blackberry.com/btsc/viewdocument.do;jsessionid=C5A7E55EABA711F3B63BE86 E36FD4E11?externalId=KB33238&sliceId=2&cmd=display KC&docType=kc&noCount=true&ViewedDocsListHelper=co m.kanisa.apps.common.BaseViewedDocsListHelperImpl
We picked disable EWS.
The above describes the process.