If VPOP3 2.x crashes when a user tries to download mail, there are a few possible causes we know of:
A virus scanner or Internet security software is interfering with the connection between VPOP3 and the email client and is either terminating VPOP3 deliberately, or is blocking VPOP3 causing another part of it to crash
Disk corruption or errors are causing VPOP3 to crash. VPOP3 accesses message files using a system called 'Memory Mapped I/O' for performance reasons. One of the downsides of this is that a disk error will cause the software to crash rather than be able to detect the disk error.
A user_<name>.db database file in the VPOP3\_databases directory is corrupted. If you can detect which user this problem is happening for, you could stop VPOP3, delete/move the relevant database file, and restart VPOP3. This will not lose any messages, but it will lose message 'flags' - eg Webmail/IMAP4 'read' flags etc.