The detoriation of the electrolytic capacitors is a known problem of the Roland XP-80 synthesizer. I have one, love it, and it started to get problems too. So I replaced all electrolytic caps in two shifts.
First all the audio-related caps. After this my XP80 sounded wonderful again.
In the second shift I replaced all remaining electrolytic capacitors of the main board. And now my XP won't boot anymore... I don't know what to do next. So I decided to ask for help here as I found some very useful information on restoring XP50 and XP80 synths. Here's a report of what I did so far:

When switching a normally working unit on, the display goes on, kind of flickers and an animated logo of XP80 comes up, after which the buttons light up and the sound engine starts.
In my unit only the screen is illuminated, flickers and then freezes.
Sometimes a number of buttons light up and go out again. In seemingly random patterns. The synth does not react to button presses.
I can provoke the same long button-light pattern when trying to boot the synth in test mode (by keeping the EFX, Utility and Rhythm buttons pressed for 10s during switch-on. Then some kind of dance of the switches starts which lasts about 10 minutes. It is always the same sequence.
And when starting the XP always the same seven top left switches light up:
Filter/Env. - Level - RPS - Arpeggiator - Perform - Patch and Rhythm. After a short while they go out.
I don't know if this pattern is some kind of error code, but I was not able to find any information about that.

I checked all cap connections with neighboring components. They are all okay. I checked the voltages and ground connections. All okay. I doublechecked the polarity and value of the mounted caps. All okay.

I also replaced all electrolytic caps of the power board. Did not help.

The problem lies probably in the CPU IC. It is IC12 on the main board. It is a Hitachi (nowadays Renesas) SH7034 family processor. I downloaded the bulky manual of this processor, but unfortunately, I cannot find any useful clues about the booting process to help me further (or it is beyond my knowledge).
All Vcc (+5V) and Vss (ground) connections on the corresponding IC-pins are okay (a prerequisite for starting the chip). A manual reset works, but restores the same freezing state.

Can anyone shed some light on this processor what to do or what to test to get this processor going?
Can it be a timing problem of some sorts? (I measured all Vcc times of the main processors. All lines reach +5V between 0.007 and 0.0095s after switch-on. So it seems, there are no anomalies there).

Could it be one a defective cap? Can I detect a defective cap when it is soldered to the main board?

Here are links to the
* XP-80 service manual: https://www.scribd.com/document/48697252/XP80-service
* SH7034 manual: https://www.rockbox.org/wiki/pub/Main/DataSheets/sh7032h.pdf

Any help is greatly appreciated!!

Kind regards,
Vincent