Turned out I had to re-adjust the tongs/jaws mechanism and springbox.
The ball joints were loose (one lockring was so seized that I had to heat it up AND put a piece of pipe over the hook wrench before I was able to unscrew the thing), I corrected it and afterwards it was a long run of adjusting and re-adjusting:
-the nuts at the end of springbox rods must touch the stops without applying force (I was tempted to change the leftwards stroke of the lower rod by tightening the nut at first, but it would be no use as the rod abuts the right tube cap),
-the ball plug c26E6E was not screwed in far enough - the ball extension a26E1 was loose in its joint,
-horizontal adjustment of springbox rrelative to the cam lever ball extension by means of a26E5 is critical. If the springbox is set too far left, the matrix jaws will close too early (while the springbox is still moving away) and you won't be able to turn the machine (it's easy to determine which lever is the culprit - take off the matrix jaw tongs). Adjusting the springbox too far to the right will result in pin jaw tongs not opening wide enough, and that's the problem I needed to solve (taking care of some loose parts and understanding my machine better by the way
Note: when standing left to the caster (facing the gears), holding the pin wrench under the bridge lever - turning a26E5 clockwise (wrench down) moves the springbox towards me (leftwards) and the distance between ball socket plug a26E5 and locknut a26E7 increases. Counter-clockwise rotation (wrench up) moves the box away from me and the distance between a26E5 and a26E7 decreases. (it's a bit confusing
I may tweak this just a little bit more to get the matrix jaw closing time a few degrees earlier (is 110°, but should be 105°).
-ball extension to cam lever height (by a24E7 locked with a24E8) - affects the length of leftward/rightward springbox movement,
-upper rod c26E17 length adjustment (screwing it into / out of the crosshead by turning the rod end nuts held together with two wrenches) to synchronize the closing of front and rear matrix jaws and pin jaws; this adjustment is easy and once you get it right, there is no need to readjust,
-pin jaw closing at 320° (1 paper at H7, 2 papers at A1); this also affects the opening at 105° and must be re-adjusted whenever the rod ball joints, springbox ball joint, rod end nuts or ball post height is touched.
I had a situation where the pin jaws adjusted correctly when close were not far apart enough when open (didn't go all the way to the end of the pin block). Remedy: forward/backward springbox adjustment.
I didn't have time to cast anything today, but the machine turns nicely, the jaws open as far as practically possible, still leaving some room for overthrow between the jaw and the row end clamp.