I have an 8300 I have just upgraded the firmware to the latest one according to this sticky.

I get this error message every time I try to send a text message;

uncaught exception java.lang.IllegalArgumentException

I go into menu then click on text/media messages then it asks if I want to [use once] or select a contact. Whether I pick [use once] or select a contact I get that error message thrown. It happens every time.

I can make and receive calls no problem.

I have had the phone unlocked, it was originally locked to Vodafone UK.

I would try to reflash the phone. Something seems corrupted.
Atleast java error indicate it. Guess you have tried to pull the battery, if still no go, reload OS again.

How do I reinstall the OS? I load the desktop manager up and it says there are no available updates.

This error was being thrown when the phone was on the old firmware as well, I thought updating it might fix it but no such luck.

Is there anything that doesn't get updated when updating firmware?

have you already pulled the battery while it's still on? then put it back in after 30s; if you have to reinstall OS, try following the instructions on this link How do I wipe the BlackBerry using Jl Cmder? - BlackBerryFAQ.com

Yes I did try taking the battery out with the phone still on, left it for 30 seconds and then put it back but still no joy.

I deleted the OS using the guide you supplied ricpac. I think everything went successful on the delete side, I got the 507 error message when trying to reboot.
So I tried to install the OS again, however the application loader wouldnt recognise the blackberry without the battery in so I put the battery in and reinstalled the OS.
I am still getting the same error messsage however.
Any ideas anyone? Thanks for the help so far.

You won't be able to power the BB without battery.

If the JL_cmder trick that ricpac posted you don't work your phone seems like it wants a warranty replacement. Is it still within warranty?

No I don't think that it is still under warranty. Do you have any other suggestions?

mutd85, what about downgrading back to the OS version you were using before the 'upgrade'?

Well your error is a software error, its very strange if your phone was wiped and you re-installed and got the same error again, did you apply the newest OS or the same version? you could try to downgrade or upgrade regarding of what you first did.

I applied the OS that's in the sticky on the top of this forum. I've just tried it with a different SIM card and the error message doesn't get thrown, which I think is kind of strange?

hmm ok that made me think a bit.

Options -> SMS
Check that "leave SMS on SIM card" is set to No.

Posting a screen of my settings, you might want to compare.
Just dont change your service center number, thats unique for each carrier

I compared my screen to yours and the only differences were Validity Period and Number of Previous Items, which I then changed to match yours.
Still the same error message though.
Thanks for the suggestion, any more ideas?

Well, since it worked with another sim card.. I would contact my carrier and get a new one. You should get one for free or a very small fee.
Depends on where you go. And carrier policy

Worth a try huh?

Yeah, thats a good idea. Thanks for all the help

Post an update when you do. I'm curious what the fix turns out to be.