Is there another place on the internet where I can get help with solving plot problems?
writing.se does accept questions regarding plot problems - and to be clear, it means they accept questions on how, not what to write. They have a tag for it, and its description goes:
Plot typically refers to the events and their ordering in a work of narrative fiction. Use this tag for questions regarding plotting, outlining, and narrative arc.
But what you want is not help with a plot problem.
I currently struggle with coming up with a plausible excuse that a detective surveiling a suspect might give for his presence when the suspect notices that the detective is following him around.
That's not a "plot problem", that is writer block. And to solve it, you want someone to come up with a part of your story for you. There is a name for people who do it professionally: ghost writers. Maybe that is the kind of help you need.
If you must ask an open-ended question, the stack model will not help you because the whole point of Stack Exchange is to find the best answer for specific problems. And that is why it is so popular: the more specific the question and the answers are, the better we can rank answers by quality. People who have had the same problem in the question will find answers easily. But for questions with no right or wrong and which need a lot of post refinement after answers appear, filtering for good answers objectively is not possible. You get stuck with the forum model of discussions and its characteristic level of quality.
If you are still willing to sell your soul to the devil, the least horrible site where you can ask is Quora. Be warned that answers might be tangential, off-topic, vague, and they will not be cleared up. The people there accept questions like "what is your favorite movie?" and "how can I lead a happier life" after all.
Quora is just the purgatorium. Reddit is where hell starts for creative discussion. Not that Reddit is bad - it's just that it is heaven for a lot of things, but not for what you want.
Below hell there is the empty void that is Yahoo! Answers. It is filled with one liners, bad advice and trolls. You might as well try to catch fish at the top of the Everest.
Finally, the most desperate place to ask are the chan boards. The most well known is 4chan. You will have a 99.99999% of chance of starting a name calling argument, and a 0.000005% chance of receiving a visit from the FBI (or equivalent in other countries) for pedophily when they post illegal content in your thread. You still have a 0.000005% of chance of getting a proper answer though. Last time I saw (in the 2000's) there was a board for requests, /r/, where you could ask for literally anything and a minority of the people there would try to reply to their best capacity. I have not checked (and won't check) to see if that letter still stands for "Requests".