Electronic brick - 125Khz RFID Card Reader doesn't work?

Hi everybody!

I have a Electronic brick - 125Khz RFID Card Reader.
A half year ago I used it with my mBed and it worked fine. I took it out of the breadboard, placed in a antistatic bag in a plastic box together with the RFID tags and didn’t use it for some long time.
Today I took it out of the box because I wanted to make something with the module. Wired everything up, and when powered up and the red led went on. But when I put a tag near the coil the green led doesn’t turn on anymore and no data is present at the uart pins of the module.

I measured the resistance of the coil: 5.8 ohms, so that looks good. When the coil is connected to the PCB there is an AC voltage on the connector. So the module seems fine too…

Does anyone have a idea how to fix this?

Thanks in advance,