I didn't find this bug anywhere on the internet, so maybe I'm just dumb and can't see something that should be clear, anyway here's the problem:

I should start the mission "Dead Man's Party" from the expansion Hearts Of Stone, so to begin the mission, I should head to Shani’s clinic on the western side of Oxenfurt, but the door is "locked" , and there's no way to open it.. is it a bug? Can I solve it someway? I'm condamned not to go on with this expansion? xD

Thanks for your help ;)

[Edit: typo]
Do you have the correct door, and have you met Shani yet? There is a prerequisite for this quest.

Thanks for answering, yes, it's the only door to enter that house, and I looked a video on youtube about this quest and Geralt enter from there, so I'm pretty sure of this. I met Shani last time during the mission with the prince frog in the sewers of Oxenfurt, where Geralt ask her to stay away from the nest, after the battle then she desappeared actually and I haven't seen anywhere, now that I think about it, after that mission... maybe I should find her somewhere first? Someone remember? ^^

Once the toad is dealt with and this quest is active, she should be at the clinic, but she doesn't open 24 hours. What time of day are you visiting? Try zoning out of Oxenfurt, rest for a day and go back around mid morning/lunch time.

I tried, even changing continent, tried every hour, tried to cast signs on the door (never know) , complete a secondary quest and return to this, but nothing changes :/

That's odd. Is there a sign above that tells you it's Shani's clinic? There should be, and it should be interactive. I'm just trying to establish if maybe your installation is corrupt. If there's no sign then somethings wrong. If there is, then I can't think why the door is locked. In any case, if you use Galaxy it may be a good idea to repair your installation.