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Strange orange stains on a fresh plaster – what is it and how to get rid of it?

Two weeks after plastering, in the corner came out this orange stuff with an unpleasant smell. Surprisingly, it's a corner of an interior wall adjacent to an apartment hallway (i.e., no temperature fluctuations). The plaster looks to have dried about a week ago. Only noticed it yesterday. Layer of about 4 cm in this corner. The wall is gypsolite. It has always been dry and there has never been any fungus in the apartment. What happened and what is the most effective way to deal with it?

• plaster
• stains