Seems the taxa page does not recognize the project_id parameter but uses a different one - projects which is inconsistent unfortunately. So your query in fact tries to list all observed taxa on iNat which is a heavy load.
The correct and working URL for you would be this:
https://www.inaturalist.org/observations/taxa?place_id=any&projects=87411
The other thread is closed so I’ll write it here. @bouteloua 's query is also incorrect and works only because there’s additionally a place_id to restrict the observations and returns the taxa of that place not of the project.