When I try to turn on the 8300 with fully charged battery, it shows a message in the center of the screen :

JUM ERROR 102

and "RESET" in a box under it.

What shall I do?

Thank you very much for your help.

Hello - the JVM 102 (Java Virtual Machine) error has been discussed a lot - I did a search on the phrase and came up with a lot of hits..

BlackBerryForums.com : Your Number One BlackBerry Community - Search Results

Thank you very much NJBlackBerry, for your lead, seems that's a little beyond my capacity of fixing it myself, I'll try send it in for repair. Best regards.

This happened to my phone too. I just press reset. The phone rebooted and back to normal. Maybe your problem is persistent?