OsCalss 8.1.1
I'm using a copy of sigma theme, ver 1.0.0, but the behaviour is the same with version 1.4.0 that i also have instaled and activated for testing.

While acessing the index page on my osclass site, the main page, there are some blank spaces left between some listings.

You can check the behaviour on this site: http://www.atol.pt

The spaces are not always the same and they change sequence or position, as ads are inserted or renewd.

Any idea if it may be a problem with the Sigma theme or something else ?

Usually happens if imags does not have same size.
you can change it in Settings > Media, change "Force image aspect", save and then regenerate thumbnails.

Previously had the "crop image" option selected.

Now selected "Force image aspect" and removed the selection from "crop image".

Regenerated images but the problem persists.

Will check with new ads/images.

Any other sugestion ?

Crop is OK.
Only important is to have all images of same size.
That's it.
In some cases it is needed to update "no-image.png" to match expected dimension.

Checked your site and images are not of same size.

Will do and check the image size of the ads.
What option will work best to make sure the uploaded images are the same size ?
Crop or force image aspect ?

Crop or disable force