Tales of Symphonia

Either your game disc(s) are scratched, your Gamecube's lens is dirty, or your Gamecube is just being fussy. I rarely get this error, but opening and closing the lid clears it up for me.

Does it happen while playing any other game?

Your disc may be scratched.
What you need to do-and I am serious-is smear a thin layer toothpaste on the scratched parts and let it sit for 20 minutes to half and hour. Then, wash it of and see if it works.
Works for me :)

If it's your game cube, you can open it up, take the disc out, and try sliding the lens thing to the back. This is not something I suggest you do however. It works sometimes, but is bad for your system.

It has happened to me a lot of times. I washed it a lot of times and it worked. Like SDWjolt21 is saying, just try washing it a couple of times. Try different ways of cleaning it, like soap and water. I tried alcohol and it worked. Now, it doesn't really happen.
Toothpaste works? I should try that.

If it starts happening with other games too, it's definitely your gamecube. A lot of gamecubes are starting to have problems with the laser. It's supposed to start at the bottom and move up during gameplay and yours might be having issues doing that. My gamecube rarely even starts games anymore unless I try for like 30 minutes.

You still have your GCN? o.O I sold mine for a Wii (cause its backwards compatible). Most likely your game disc is scratched and you may need a disc fixer (should be in most local video or game stores). However the problem also could be the Gamecube itself, possibly the lens is dirty and you may want to try cleaning it with a kleenex and polishing agent. be sure not to touch it directly or you may damage it. If this happens constantly, you may want to consider bringing it in for repairs cause once a console is a pretty old, it starts to have problems. My N64 recently stopped working, most likely cause its pretty darn old (like 10 years about) so you may have to repair it...
here's a link in case you still are having trouble and are considering getting it repaired (its a nintendo link, so its genuine): www.nintendo.com/consumer/repair/repair_charts_gcn.jsp