I had these issues too a week or so ago.
Just because a model you have created ‘looks’ solid to you …the software dos not see it as a ‘solid item’
Solid has a specific meaning in relation to all these 3D stuff… it means more that what you might expect in standard english.
Stray lines or odd looking faces mean it is not ‘solid’
Best advice I had for that was set the model to 'MonoChrome" View in Face Style.
Turn off X-Ray (if you had it on)
Then look around the model, right clicking the odd side here and there and ‘Hide’ …then you can see inside and see random lines and faces.
the model inside has to be totally hollow, for the software to class it as a solid…if there is a Face or line blocking one hollow part from another it will not be ‘solid’ so the printer software ‘screws up’
Solid really seems to mean “continuously hollow”