I realize this is an older topic, but there is no resolution, on the thread, except the usual light-hearted banter about buying a different make of reel, take it to a service center, etc.
With the new season either on, or fast approaching depending on where one lives, this will come up again.
Of the thousands and thousands of Abus fishing, this scenario does come up, but not s often, as some would make out.
There are four main causes of the problem. Not in any order:
a warped side plate.
lack of lubrication to the clutch arm, and the casting trigger.
normal wear and tear to the clutch arm.
lack of, or unbalanced spring pressure from the position holder.
In the image of the clutch arm, the guy opted to fix his by altering the radius where this part contacts the brake plate post. The usual fix is a slight "re-bend" of the very end of this part... that little 90 degree'er out there. Over time, the tip wears, and does not engage the driveshaft teeth too well. The re-radius does much the same thing: permits the clutch arm to come away easier.
The clutch arm is inexpensive, if re-worked past the point of useability..either by a bad re-radius, or a real bad re-bend of the tip.
The tip only needs the bend, not the main part of the arm. It needs to stay flat along the brake plate. They are about 4-6 dollars. I normally do not re-radius, just bend. Haven't lost one yet.
The position holder is also an inexpensive part, and if ruined in tension adjustment trials, costs about 5.00, or less. I have new on hand, and if I wreck the already wrecked old, on goes the new. The two stainless steel "prongs" will have too much, or unequal pressure preventing the clutch arm from coming back out. It's trial and error to get the right tension on each arm, of the right pressure. New is better. But it's nice to do a fix for ones'self.
These I have lost. Maybe 2 or 3 over the past 10 years. Hope this helps. Best, KD