An internal error has occurred. Daylight simulation has been aborted.

This article is applicable to DesignBuilder v7 and v2025.1.

1. Check the DesignBuilder session logs
https://designbuilder.co.uk/helpv2025.1/#Diagnostics.htm 

2. Search for the reinsrc.cal in the last session log file:

Daylighting::DoDaylightingCalcs - Command 'cnt.exe 577 | rcalc.exe -e MF:2 -f reinsrc.cal -e Rbin=recno -o "solar source sun 0 0 4 ${Dx} ${Dy} ${Dz} 0.533" >> "C:\Users\LuisS1\AppData\Local\Temp\dkl3athd.imv.Building-1.geom_sun"' failed to run: reinsrc.cal: cannot open

3. You may have installed the standalone Radiance or other software that is using Radiance at some point. Some software overrides the Path and RAYPATH, breaking the Radiance mechanism that DesignBuilder uses.

Solution steps:

1. At Control Panel > System > Advanced system settings > Advanced (tab) > Environment Variables > Under "System variables" and “User variable”, find RAYPATH and delete it.

2. Delete all Daysim and Radiance references on the Path variable at both user and system level.

3. Start a new DB session and try Annual Daylighting.

4. If the issue persists, copy reinsrc.cal from DesignBuilder's own Radiance folder (C:\Program Files (x86)\DesignBuilder\Radiance-5.2\lib) into every other Radiance installation on the PC. You may need to search your machine for other applications that also install or use Radiance (e.g. Daysim, standalone Radiance) to find all the locations that need the file.