I take you've been riding in the rain?
As with most things on the stick with the simple
.
If the above(rain) is correct I stick with the front cyclinder. It can be for a while and when you stop or in your case pulled the clutch The moisture/water had a better chance to do a good job of cutting the spark- Trying to describe it but my brain ain't working properly.
Which grease did you use as if not rubber safe you may have made the boot swell up and so not doing it's job properly. Depends on how good a job you did as well. Plug hole covered.
Basically I'll put money on the spark plug. Make sure you clean the grease of around the spark plug lead are to prevent crap getting in betfore youattempt to remove and clean/replace the plug.Check the drain hole if clear.
Ben