Unable to scan JTAG chain, TCK only 2V

I have trouble with some development board with 10CL006YE144G. While the configuation by JTAG worked a while, the FPGA is no longer recognized by the programmer using Autodetect. 

I checked the voltages of signals and while the power supply (JTAG Header Pin 4) is stable 2.5V, the TCK signal is only driven to 2V High level. As all I/Os are driven by 3.3V, I assume the 2V is not or at least no stable high signal. As two USB Blasters do not work / work on other designs and with the signal is 2.5V w/o connected development board. I'm afraeid the FPGA I/O is broken... 

Unfortunately, the dev-Board lacks protection devices (direct connection header to Pin, no protection diodes). I assume connecting the USB-Blaster being referenced to the PC to the board being referenced to board's power supply could cause voltages spikes on the interface overloading and damaging the FPGA :-(

Is this a keep the board simple and cheap, i.e. not even a series resistor, trap?

Thanks in advance and KR Carlhermann