In this case after investigating is the correct way to say it.

Your answer is complete and the best I think, Unfortunately, your answer is not the type that I could select as the best answer. It's type of comment though.

Hello! "Investigations" implies that there is more than one person. For example, "Investigations into the crime took months to complete." "Investigation" is more neutral - it could be one person or many people. For example, "Upon further investigation, I discovered that my cat had eaten a sock." "The police investigation was inconclusive." In your example sentence, it could be used like this: "I had an issue with sending a request to the endpoint, and after some investigation, I noticed what the issue was." Hope that helps! Emily