I have had that problem occasionally for almost a year now.  Evan Lewis (Khan Academy Support Advocate) has been collecting information from people who experience that problem so that they can (hopefully) eventually figure out what is causing the problem and how to fix it.  I recommend reading my post about it (see link in next comment, or look through my posts here in the Support Community) and posting the details in a comment on that post with a tag to Evan Lewis.

~ Janice

https://support.khanacademy.org/hc/en-us/community/posts/35231517036813-Is-anyone-else-bumping-into-the-rate-limit-for-posting-even-though-posting-fewer-than-100-times-day