According to my investigation "Output trip error" meas the output is disabled because an error is detected.
One error could be the over temperature, but it is unclear what are all the possible errors leading to an output trip condition.
Unfortunately there are no details about these in the manual.
@TTI Tech: It would be nice to have a little more info here...Example, if I use my other PL303 as DC load (with a small current just for testing purposes), I can reproduce the output trip error condition that can be cleared once I disconnect the inputs and press the output button.
That said back to my unit.....somewhere in this control board there is a problem.

Some tips for future repairs:
1) The left leg (see blue circle) of the big "Mexico" R79 is GND
2) There are many SMD fuses (very robust design) to check "FBxx" (see yellow circle as example)
To the best of my knowledge all the analog input and output sections are working properly.
Since the uP goes in output trip error immediately after power up, it is detecting something is off and the output are turned off.
By pressing the output button there are no changes in the output analog sections, which tells me the problem is not a consequence of a bad output state but something is the inputs independently from the output state.
Some of those inputs are processed directly by the ADCs in the uP PIC (like the over temperature analog signal) and some go through the IC7 ADS1241 ADC.
The display shows OPT Trip so I tend to assume the uP PIC is good. So if I have to bet, the IC7 ADS1241 is the bad guy.
I could just take and install the known good IC7 ADS1241 front the other control board, but first I want to be more sure about my theory.
The next days I will try to dig into the digital signals I2C, clocks and so on....
I will now also contact the awesome TTI support and see what they say.
PS: There are also the programming pins for the uP available (PJ6), does it make sense to connect a PICKIT to diagnose the problem? Sorry for the idiot question I have not played with PIC uP too much.....