Definitely the handheld's Low Memory Manager at work due to insufficient space...
KB-02843 What Is - Low Memory Manager
Livelink - Redirection
Excerpts:
The Low Memory Manager is activated automatically when the available flash memory on the handheld decreases to less than 400KB. Flash memory is used to store handheld applications such as Messages, Calendar, Address Book, Tasks, MemoPad, and third party application data.
By removing rarely used applications and old data you can increase the amount of memory on your handheld. The Low Memory Manager begins by removing low priority items such as cached data. Medium priority items such as very old emails and out-of-date calendar entries are deleted if additional memory capacity is required. If the memory situation on the handheld becomes critical, the Low Memory Manager will cause the email application to remove emails on a last-accessed basis.
To view the available memory on your handheld go to:
Options > Status > File Free
Note: Calendar entries and emails removed by the Low Memory Manager are not reconciled back to the mail system.
I would remove any 3rd party apps or less frequently used apps on the device to give way to the more important messages...