Easy to replicate. Take working 1.4.21 instance, and updgrade PHP 5.2.13 to 5.2.14
Here are my diagnostics:
SquirrelMail version: 1.4.21
Config file version: 1.4.0
Config file last modified: 11 May 2010 02:53:33
Checking PHP configuration...
PHP version 5.2.14 OK.
Running as N/A(N/A) / N/A(N/A)
variables_order OK: EGPCS.
PHP extensions OK. Dynamic loading is enabled.
Data dir OK.
Attachment dir OK.
ERROR: Set User Data plugin is configured with an unknown data handler ''
ERROR: Some plugin tests failed.