Hi, hmm can you let me know which widget are you activating (Adverts Categories or Recent Adverts)?
Also, do you have any plugins related to Widgets? If you do then you can try disabling them one by one and see if it helps.
The error suggests there is some blank condition added to the query but i am not yet sure where does it come from.
This is to update you on the issue.
It turns out that the main culprit is [search everything] plugin https://ww.wp.xz.cn/support/plugin/search-everything/
after disabling the plugin everything went smoothly as expected with WPAdvert and other plugins.
Thank you for your patience.