Well there is a good and a bad news :
The good news is, you we right about the PA. With the help of Sharpcap I managed to reach nearly 10 minutes exposure @ 300mm unguided ! :O
The bad news is, I'm afraid something is wrong or broken with either my mount or my camera : I wasn't able to calibrate either PHD2 or Kstars for guiding tonight. Didn't matter the platform (ASCOM or INDI) nor the software. It just seems that the mount did not respond to the slew commands. Either that or the ST4 connection between the camera and the mount is to blame
PHD2 kept screaming "RA Calibration Failed: star did not move enough" at me, and Kstars reported a similar issue. And indeed, I could barely see stars moving on the screen during the calibration process.
Any idea on how to diagnose where the problem is coming from ?
That's great to hear, you are making great progress! I don't think there's anything wrong with your mount. I had the same error when I tried to guide for the first time. I think it was because I had accidentally tossed the mount and lost polar alignment. When I tried the second time, everything was OK. You really don't see much movement, because ST4 guide speed is 0.5x by default, and SGP is constantly tracking with 1x speed. Every time SGP receives a guide pulse from PHD, red indicator light blinks on the SGP. If I recall correctly, you can test this with PHD manual guide tool. Bear in mind that you won't see any movement on the screen, but if the indicator light on SGP blinks when sending a command, your system is probably functioning correctly.
The only way I found to verify the guiding is to use ASICAP software. My guidecam is ASI, and I am not sure whether ASICAP works with your guidecam. But in ASICAP there is very simple autoguiding tool, where you can click arrow buttons to send guide commands and watch the camera output at the same time. If you set the guiding speed from SGP to 0.5x, you should see the screen slowly rotating to east. Now, if you keep clicking in bursts to west, the screen should stay still. If you keep clicking in bursts to east, the screen should rotate with double speed to east. I did this test indoors for convenience, and had only the guide scope on the mount (attached with scotch tape).