I finally found the root cause - Applies for FC9225, but may also solve other types of the same family.
The problem is the switch for the dustbox (not sure if the hinge is worn or the sensor (contactless switch) has another issue). Removing this safety switch (allowing you to switch on the vacuum cleaner without dustbox and filter cartridge properly installed, which I judge not a major 'risk'), permanently solved the issue, 1. Remove the lid 2. Remove the big sealing ring on the bottom of the lid 3. Pull the cantilever switch out (and dispose ;-) ) 4. Re-assemble 5. Fixed the problem in my case
(check 4 last images I added)
For your entertainment: I also filled the connector at lid side with glue gun glue, de-soldered all capacitors and measured them, completely dismantled the motor, pulled the brushes and cleaned the commutator... to find it was just the ****** switch...
DISREGARD BELOW BUT USEFUL FOR DISMAANTLING
We had similar issues (all power level lights start flashing when connecting). We could extend lifetime by electric contact spray. But also this stopped working at some point. Today I finally dismantled. Tip 1: to remove the lid, just push the 2 plastic covers outward at the hinge (no need to dismantle the lid) and push out the shaft.Tip 2: when dismantling main body, unscrew the small wheel below the 'dirt box' (OPPOSITE DIRECTION = CLOCKWISE TO LOOSEN). Tip 3: only dismantle the main plastic cover by loosening all screws.
Then dismantle the main body connector, remove the rubber block at the backside of the connector, make sure the 4 wires are as deep as possible in the connector, fill the connector with glue gun and put a tie rap to keep 4 wires together.
If you want to dismantle the connector, you need to push inside the 4 small latches near each wire, BE CAREFUL NOT TO ROTATE THE INNER OF THE CONNECTOR 180DEG WHEN RE-ASSEMBLING (there is no poko-yoko to avoid this).
I added some pictures to hopefully make things a bit more clear.