Ok some electrical information:
Space heaters are typically 1500 watt, about 12.5 amps as I recall without doing the division.. Confirmed by mutliplication.
Outlets are either 15 amp (Two parallel slots) or 15/20 (one slot is "T" shaped)
Switches can be 5,10,15,20 amp,, 20 amp are rare, even 15 amp are kind of rare
Nest there is that good old ROUND hole/pin. the safety ground.
If the heater has a ground fault then that pin may become HOT (Hot to ground short) and this will make the switch box, and the switch itself HOT as well, if there is a break in the safety ground between the switch and the service panel, or a bad connection (SUSPECT #1) in the swich box.. I can see what you saw.
Recommendation:
Check heater, an ohmmeter measuring between the flat pins and the round pin may be good enough to find a ground fault, if such exists REPLACE if you do not know how to repair.
Inspect all connectins in the switch box for signs of arcing, tighten or better yet replace teh switch (Don't take the chance) with a switch rated at least 15, better yet 20 amps from your local hardware or electrical supply Check the box to make sure it can haul the freight.
ALso do a check to insure a good solid ground,, now this one is tricky. You might want to have someone do it, But what I'd do is make up a 100 watt test lamp...
The neutral slot is longer than the hot
Use probes
Test between HOT and Neutral, notice the brighteness of the lamp
Test between HOT and GROUND, Same brightness.. GOOD, Dimmer or arcing in the switch box BAD Very Bad.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times