Turns out that Codeforces Talks has a strict limit on the number of unique users you can message per hour. The limit is 2 :/

But there also seems to be a strict limit on the number of unique users you can message per day. That limit also seems to be 2, but there is some bug in its enforcement. Specifically, today I somehow managed to message 3 people, but now any message I send (both to new people and to people I've already sent a message today) gets blocked with You exceeded your daily quota of 2 distinct recipients.

Sample repro:

time X - send message to user A
time X - send message to user B
(now you're blocked from sending messages to anyone but A or B for 1 hour)
time X+1hr - send message to user C
(now you're blocked from sending messages to ANYONE AT ALL)

P.S. Apologies to anyone who sent me a message in response to my testing request and hasn't heard back — I'll (hopefully) respond tomorrow.