Select the Devices icon (the circle with the lines, the target icon), then the Electron you want build for, click the “>” icon, and verify that the build with firmware popup menu has a valid value. The setting sometimes get set to something odd, which causes the Error 500.
If you have not upgraded the system firmware your Electron should have 0.4.8. There is a bug where some subset of Electrons show as having 0.4.9 on the device when they really have 0.4.8. This causes Error 500. Manually selecting 0.4.8 (or upgrading to 0.5.3) solves the problem.