ST-Link Can't Read Memory

Hi everybody, I recently bought a STM32F103RBTx and it worked perfectly at the first run.

Today I tried to use it again and I meet a lot of problems.

When I launch the ST-Link Utility, I have a message saying " Can not read memory! Disable Read Out Protection and retry."

Then I go to Target > Option Bytes and I set "Read Out Protection" to "Disabled". When I press apply I have this error again: "Could not set Option bytes! Please reset the target and retry." I close the message and the following one appears: "Can not connect the device".

I really don't understand... I'm a beginner with STM32, I did a lot of researches but nothing helped me...

I would be grateful if you could help me...

Edgar