I decided to deactivate the plugin because my problem appears to be a long-standing issue. Back in 2012, someone else described a similar situation when using /%category%/%postname% and then removing the category base.
This and this may be relevant too.
Being a newbie to www.ads-software.com, I had the choice between moving away from /%category%/%postname% or deactivating the plugin. I decided to deactivate for now. Thank you.