Is it allowed to ask for help in community for thesis? I'm writing on my master's thesis. A specific part causes some troubles and I hadn't any progress here for several weeks.  

I'm now considering to describe my problem, my ideas and my accomplished work so far in detail in a community and ask there for help.
Would that be plagiarism, if I got some help from the community? So I would be using someones idea.

The thing is, my problem is very domain-specific, so I can't really generalize it and ask at anonymously.

My adviser already tried to help me, but still I'm not really making progress.
Plagiarism is when you use other people's ideas and fail to give them credit.  So if you make it clear that an idea came from someone else, and credit them appropriately (with a formal acknowledgement or a citation), you will avoid plagiarism.

Of course, in order to be acceptable for a degree, your thesis will still have to contain a sufficient proportion of ideas that are your own.

Before discussing the details of your work with others online, you should ask your advisor whether it's a good idea to do so.  It's possible that your advisor specifically wants you to work out certain parts for yourself.  Another concern is that sharing your ideas increases the risk that someone else will carry out your research before you do - in most cases, this risk is outweighed by the benefits of sharing, but your advisor needs to agree.