BizTalk date handling gone spooky

This happened around this time last year too...

Suddenly we started receiving errors around the parsing of dates in our production BizTalk 2006 environment. BizTalk could no longer successfully run this code:

DateTime d = Convert.ToDateTime("14/04/2010");

String was not recognized as a valid DateTime

Somebody tinkering with regional settings? No, as a hard-coded app with the same values run under the same user on the same box produced successful results. Only one Application was experiencing the issue.

More strangely, it appeared to be only happening with today's date. Other dates (e.g. 20/04/2010) worked successfully, so no issue with not understanding UK date formats.

I'd like to say it's due to X, Y or Z, but alas restarting the App Host in question cleared the issue. I've seen it once before last year, bugger knows why!

Anybody with a clue, please feel free to contact me.