I regularly check this thread in the hope of some return to booking SQ awards, which was one of my favorite ways to spend UA miles.
Today, for the first time, I saw a nuance in the language that United has posted about this problem:
Award travel on Singapore Airlines is temporarily unavailable. Our airline partner is diligently working with us to address this situation.
Note that they claim their airline partner is working diligently with UA to address the situation. There is no mention of how diligently United is working with SQ on the problem.
I think a resolution to the snafu will require
both parties to work diligently, and that does not (yet) appear to have occurred.