As Gyro has just pointed out, it looks like whatever goop that was used to bond the core in place has degraded and become slightly conductive.
To have any hope of fixing it, all the degraded bonding compound on top of the shell the winding is potted into must be removed without gouging or cracking the shell, (but don't dig down between the core and the inner tube of the shell), and the shell surface must be thoroughly cleaned to remove surface contamination. At that point, you can try photoing it running in the dark again - any 'hot-spotting' of any residual discharge is a really bad sig, see below. Then to have the best chance of a successful cure the flyback must be baked at close to 100 deg C for an extended period to drive out moisture before an attempt is made to seal the surface with epoxy or dope, following the manufacturer's exact application instructions, and once dried/set, I would recommend post-curing at an elevated temperature (less than the max permissible continuous working temperature of the coating) to ensure as complete a cure as possible for epoxies, and to drive out any solvent residue from dopes
Even so, it may be too late as if any carbonised paths have developed through the shell, there will be sufficient electrical stress at the point you attempt to seal over them to degrade the new insulating coating till it breaks down.
Corona dope, or any product sold as a sealing spray for EHT cables etc. e.g. spark plug cables in car ignition systems, *MAY* work.
A pro-grade laminating or electrical potting epoxy mixed precisely to manufacturer's instructions, should work. Contact local specialist epoxy suppliers to see if you can get a small trial pack of something suitable (rather than a 50 Gal. drum of it!) Any hardware store epoxy is likely to fail due to slight errors in the mix ratio leaving polar unreacted monomer, or due to conductive fillers. If that's all you can get your best bet would be a slow curing clear epoxy.
Hot glue absolutely wont fix it and will probably make it worse, as its prone to surface tracking and carbonisation that melts its way down into the mass till the bulk becomes a conductive goo.