PCL XL (aka PCL6) is a highly structured language, and hence most corruptions of the print stream can be detected by the language interpreter in the printer.
The message is implying that the print stream is corrupt at operator position 68547 (which is going to be a long way down the stream); the IllegalTag error, together with the value of 0x1 (hexadecimal 1) is just telling us what data is being objected to; KERNEL is the main sub-system of the language interpreter.
The suggestion that your print stream is being corrupted (i.e. junk is occasionally being sent to the device) is reinforced by your other symptoms ("every few minutes the printer spits out a page with a couple of symbols in the top left corner").
Check the USB cable and connections; if possible, try it with a different PC.