I have an update on the problem, which is now solved.
The culprit was indeed the thiourea toning step, and, more
concretely, the tap water I used to prepare the working
To check my hypothesis, I prepare eight identical copies, changing fixing time,
washing time and providing to half of them a final rinse of deionized water.
At least one of them was processed as I usually do.
Once they were ready, I toned them. But this time I used deionized
water to prepare the working solutions of the thiourea toning and
added an acid stop bath after the thiourea and before washing.
Finally, I toned all of them with gold, and none of the copies had problems!
I imagine that the problem was that the thiourea toning bath is very alkaline
(with a large amount of sodium hydroxide) and Barcelona's tap water is
very hard. Deionized water took care of the problem. For the same reason,
I included also the acid stop bath: in this way, when the copy reaches the
washing water, has a more or less neutral pH.
Anyway, the lesson here is that if one mixes her/his own chemicals, is better
to use deionized water for both stock and working solutions.
Thanks again to all!