Read through the diagnostics tutorial

A few things I would check first are;

1. Check the fuse on the EZB (between the regulators)
2. Check the power supply, or use a different supply - make sure the power supply is actually giving a voltage of between 5v and 17v dc.

1. the fuse is fine
2. i have tried all batteries i have, all are giving a voltage of 7-9 volts.

I have followed the diagnostics tutorial you linked. The bluetooth part is in order. On the next part tho, i measure only 1.3 Volts.

Again: as soon as i connect batteries, the leds do not light up.

Give the barrel jack a good wiggle and double check your connections to the barrel jack.

the problem is not the barrel jack. I can measure the battery voltage on the back of the board where the barrel jack is connected. Also there is 5 volts between Gnd and 5V for all the ports D0 - D19 etc.

Also measured on the pins where it sais MCLR: The Vin is correctly giving battery voltage. The 5V is giving 5V (or 4.9) The 3.3 is giving less then 0.6V tho...

And now i have popped the fuse by accidentally connecting pins that i should not have with the volt-meter...

Watch this video at the 2:00 mark and forward: https://synthiam.com/Tutorials/Diagnostics.aspx

*Do not perform the tests prior to 2:00 because they do not pertain to your issue.

Do you have power on the pins that the video recommands you test?

Also, you may wish to provide a clean bright lit photo of the EZ-B bottom. Perhaps there is a rare solder short.

I have just replaced the fuse. I had already done the diagnostics when RichMR2 suggested it.

As i have stated before: i am getting 1.3 Volts on the suggested pins after 2:00 in the video: Not 5 or 4.9 like shown in the video.

This is with a 7.2V Ni-Cd battery - actually giving 8 Volts to the barrel jack. Like i said before: many or all pins that should have 5 Volts do have 5V. Maybe one of the voltage regulators is broken? The one furthest from the barrel jack, VR2, seems a bit loose.

I cannot make a photo of the board at the moment, but as it has worked before, i do not believe its a solder short.

You might consider applying some solder flux paste to the connections of the loose voltage regulator and reflow some solder to them.

Yea a cold solder on a power or ground will cause lower voltage output due to the additional resistance.