2 LEDs lit at the same time causes reboot

[Apeland]

Hi to all!

I have a strange problem with my DOUT board. It is a combined 2xDIN/2xDOUT board made on "hole-pcb" and is veeeerrry tight arranged (I wanted to save space, bad idea ;) ). I soldered this about 3 years ago and it worked fine. I didn't use it for a year now and some solderings seem to have become old. I found some of them and got it to work just as it should.

When I switch on MIDI Thru in MIOS Studio and push a button the according LED lits, but when I push 2 buttons at the same time MIOS reboots.

I touched every solderdot with the soldering iron but it didn't change. How can I look for such an error?

Thanks in advance