Turns out I'm not the only one getting frustrated because of Hootsuite's latest bug.
Since yesterday, when you try to schedule a message, the Hootsuite send button will sometimes just "hang there". When it happens you aren't able to send any other messages for at least a few hours. Logging out and back in again doesn't help. I hope Hootsuite fixes this quickly.
I am a long time Hootsuite user and I like their product, but I hate how their constant (mostly cosmetic) "upgrades" always end up breaking one or more things.