Symptom: Was changing gear when gearbox indicator on rev counter start flashing and beeping. No gear change possible and had to coast to a stop by roadside.Over the years, there have been many things wrong with my car, some of which I have investigated and fixed.
I meant to post these findings as a reference in the hope that it may help someone (and myself, because after a long time I might not remember it anymore). It seems that some of the problems are not very common (but some are, I think).
Cause: Some weeks earlier I had noticed oil weeping out from the cover circled in red in the photo below:
This is a cylinder with pistons inside that control gear engagement. The cylinder ends are covered with aluminium caps that are held in place with circlips. The circlips expand into grooves in the cylinder preventing the cap from being pushed out. As it was only a slight weep, I decided to leave any intervention until a later. Unfortunately, it went catastrophically wrong before that.
First thing I noticed was that there was a trail of liquid on the road leading to the car. Diagnostic confirmed lack of oil pressure. Examination of the actuator showed that one of the cylinder end cover was missing. Luckily it was the nearside cover which is easily accessible. I suspect the circlip has failed and the hydraulic pressure pushed the cover out followed by an almost complete lost of hydraulic fluid and pressure.
Note that when this occur, the pistons in the cylinder will likely be pushed to the side where the cover is open and car will be forced in gear. Also, without hydraulic pressure to actuate the clutch, the engine will stall as the car comes to a stop.
Remedy: Obtain replacement cover and circlip. Replace items, fill and bleed hydraulic system. Others have fixed this by welding/epoxy gluing/permanently fixing the end covers in place. I actually fixed it by roadside to get the car mobile again, then later removed and stripped down the actuator to replace all circlips and o-rings.