I have a problem that I have not been able to solve involving glare. I shoot a lot of concert photos from 6-12 feet away. I keep getting glare from the lights. I have used a hood, filters, various ISO, shutter speeds, etc. I use a Nikon D500 camera and most often a Sigma 17-50mm 1:2.8 EX HSM lens. Do you have any suggestions that might help with my problem?
I have used a hood, filters, various ISO, shutter speeds, etc.
Remove the filters. That won't eliminate flare, but it will eliminate the portion of it that is caused by reflections off the back of the filter. This type of flare is often called ghosting. (See also, questions tagged ghosting)
When you are shooting in a mostly dark environment with a few very bright light sources in the camera's field of view, flare caused by reflections of those specular highlights bouncing around inside your lens is to be expected. In the digital age, this is exacerbated by the high reflectivity of the imaging sensor and the near perfectly flat filter stack that is in front of it when compared to the reflectivity of film that was not usually near as flat in the camera.
Newer lenses created for digital cameras tend to have more lens elements with coatings on both the front and rear of the elements, while lenses created in the film era tended to only have anti-reflective coatings on the front of some elements.
But even the best lens coatings can only do so much. Increase exposure enough and you'll see flare with very good lenses.
Post processing and adjusting contrast can do some reduction in the appearance of some types of flare, but the changes in contrast may also negatively affect other elements of the images.
Sometimes it can be as simple as changing your shooting position just a little so that the light from a specific source is not bouncing around inside your lens at the same angles.
Here's a more general question about shooting concerts and in theaters. The only answer has several links to other related questions here that touch on some of what we've discussed above. it also has a few example images.