The first time, I have beem in touch with 2 guys of the support. They took the hand on my computer, tried to change restrictions, created a new shared folder but nothing.
I sent them the NAS logs for a deeper analysis, but they didn't give me a feedback regarding those logs.
Few days after, a new firmware was released. I installed it and surprise, the access to shared folders was granted.
But unfortunately, the upgrade I did this morning - 1.82 - has broken permissions once again.
There is no problem with the web UI. I can log as admin.