Thanks Skimask,
I have re-checked the setup of LVP both in the software line #pragma config ... and in the config bit table of the WARP13a application window for it to be unchecked/not selected. I have also tested the Vpp while programming and is 12.75 volts, clear and stable. The line RB5 is floating, must I pull it down even if I do not use LVP?
Any idea about why the pic let me to program one time and no more?

I read that some other pics showed this problem with MPLAB compatible programmers, but I have tried all the receipes (1-10K loading MCLR to GND, C 0.1µF from Vdd to Vss, ALL Vdd pins joined, ... ) and nothing seems to cure the problem.

It can not also be programmed neither the config bits nor the ID word of the PIC. Curiously, the first time (right after solder the part to the board) the programming was perfect.

Maxi.