At launch of Reason the following folders are scanned for VST plugins:
C:\Program Files\VSTplugins\ (VST2 plugins)
C:\Program Files\Steinberg\VSTplugins\ (VST2 plugins)
C:\Program Files\Common Files\Steinberg\VST2\ (VST2 plugins)
C:\Program Files\Common Files\VST3\ (VST3 plugins if you are running Reason 12.5 or later)
Library/Audio/Plug-ins/VST (VST2 plugins)
~/Library/Audio/Plug-ins/VST (VST2 plugins)
Library/Audio/Plug-Ins/VST3 (VST3 plugins if you are running Reason 12.5 or later)
The user (~) Library folder is hidden by default. To open it, press the Option key while opening the "Go" menu in Finder, then select "Library".
If some of your VSTs are crashing when scanned, Reason will try to disable the plugin and skip it during scanning on the next launch. It's possible that multiple VSTs might crash, so please try to launch Reason over again and see if the scanning continues.
Alternatively, move your VSTs out of the scanned folders and launch Reason. Add the VSTs back to the folder, some at a time, then relaunch Reason to have it scan the folder again. Do this until you can pinpoint which VSTs are causing the crash.