I have not seen this message before, and I have more SP now than ever before. Neither have I seen multiple people suddenly posting about the identical problem.

For these reasons, simply not having enough SP to transact does not seem like it is likely to be the reason. Due to the occurrence on multiple accounts at once, and to my SP being at least 10 times what I have read is the minimum necessary to transact.

Therefore I am still thinking that there was something else at play.