I had a similar problem and it was the brake light switch shorting on the forks after i had moved the lever to get a better riding position. The way i found it was to replace the fuse but before turning the ignition on pull apart all the connectors for the front loom which can be found just behind the head stock in the black box and then if the fuse does not blow replace them one at a time until the offending wires were found.
Best of Luck.
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MC21 300
Aprilia RS250 track bike