We just did 3 DNN 7.4.x to 8.0.4 upgrades and one of them suffered something similar.  It was one of our oldest DNN installs so it had a bunch of old Extensions and also its skin was based off of a skin that was trying to use SolPart which was removed in the 8.x branch.  To resolve it we first removed all extensions that were no longer in use, upgraded extensions to the latest version, and then built a new skin based off of a DNN8 skin template.  The extensions were just good practice but the skin was the actual culprit.  You have to have a non-SolPart (or any of the other deprecated/removed providers) using skin in place otherwise things will go bad afterwards.  We wiped out the old skin in the DNN 7.4.x install and replaced it with the new DNN8 based one as part of the upgrade step.  It did error in DNN7 but that was because it wanted DNN8 components it didn't have yet but that was ok as the step right after putting the skin in place was upgrading to DNN8.  When makign the new theme make sure your names match up with your previous ones for the skin & container otherwise you'll have to hunt down every little thing.