Make sure you're using 5.14 version (the fix is only from that version).
In "CONFIG" -> "Captcha" -> the option is enabled to show pop-up when a special captcha needs to be solved.
Make sure there is no "red lock" icon near the status -> if there is you have bad settings either auto-retry or smart login is disabled (that would cause for the window to be shown on each post rather then just sometimes).
Open older version (or wait a bit if running) and press "Update now" when asked about the new update.
v.5.20
Added new options in "WEBSITES" -> "Other settings": "Create threads add extra flood delay (in seconds)" and "Reply to topics add extra flood delay (in seconds)".