Its just a bad sample with low amounts of dna and lots of contamination.
Normally when you shake the sample, pipette up and down a few times and then load 2µl you can sometimes get a measurement.. but its a bad sample anyway...
(the bubbles part: can be that you also just placed the drop in a "bad way", but most often its just the sample itself that is very bad)
What contaminants can we possibly get from a plasmid miniprep? Maybe some bacterial protein?
just "dirt", it can be anything... if something went wrong with the filter (in the tube) , organic matter, ... or the media you used (not removed enough) or just that you don't have enough plasmids (bad selection)..
And some proteins yes, but that should not be a problem normally.
Its hard to tell to be honest, these things just happen once in a while..
perhaps I should not have said: low amounts of DNA and lots of contamination... but low amounts of DNA and/or lots of contamination.