I have encountered the same html error issue lately using the desktop site -- the problem seems to be that pasting a message from another source can contain html information hidden in the text; the posting software will remove the html after generating an error message, but trying to post the same message more than once now triggers a spam filter. This seems to be much more sensitive since the most recent board upgrades.
Possible workarounds: paste text into a search field or text editor to strip out html first, then copy the pure text and paste that into the message body, adding additional formatting if necessary, to avoid having to resubmit the reply.
Or, if you run into the spam filter -- open the original message in a new tab, create a new reply, type in some placeholder text and post the reply. Then use the "Edit Reply" feature found under "Options" to reopen the message and paste in the original text within fifteen minutes of posting. This should avoid the spam filter.
