Rotax 912 Uls clicking sound

Is it exactly every quarter-turn of the prop or do you get more like 5 clicks per prop revolution?  (5 per 370.6° of rotation)

That would be consistent with one click per half revolution of the crankshaft.

You may be hearing a connecting rod reversing from Compression to Tension. (Push<>Pull).

If it has any runout it might be noticeable.

Check to see if the cylinders are at Top or Bottom Dead center just as it clicks.

If not, it could be something related to the Stator.

A loose coil could have moved and is rubbing a magnet.