May or may not be relevant. I noticed that the setscrew hole is not lined up with the flat (as shown by the setscrew on the spindle knob), There isn't any marks visible on the spindle through the setscrew hole, but it is possible that a burr has been raised on the spindle.
A similar but different problem came up last week on the facebook forum. A new owner was having difficulty removing a drill chuck that was stuck on the spindle. In that case he was unable to loosen the setscrew. As it turned out the setscrew had loosened a bit and then the chuck had turned on the spindle jamming the setscrew against the edge of the flat. The solution in that case was to tap the edge of the setscrew hole with a hammer and punch till it turned back in line with the flat which freed it up.
10ER #23430, 10ER #84609, 10ER #94987,two SS A-34 jigsaws for 10ER.
1959 Mark 5 #356595 Greenie, SS Magna Jointer, SS planer, SS bandsaw, SS scroll saw (gray), DC3300,