Ok. Here's a newer version that tweaks the extension loading code a bit and adds some additional debug logs. If it still doesn't work on Android 5, please attach a copy of the logcat so I can take a look. This version is 2.73ad2.
Now I didn't get an error and the extensions are where they should be: Computer\Galaxy Note3\Phone\Android\data\edu.mit.appinventor.aicompanion3\files\AppInventor\assets\external_comps
but no Button and Label are displayed and I get this error:
So based on the logs it looks like it's failing to create a specific directory on that device. It's odd that it's trying to do it on the external SD card directory. Do these devices have physical SD card slots?