Comment 2 for bug 2018187

Hmm. I'm wondering the file /etc/grub.d/bin/grubcfg_proxy is executed when launching grub customizer. Generally it's reading the original scripts bypassing the grubcfg_proxy.

It says the script /etc/grub.d/bin/grubcfg_proxy became incompatible. That's a problem of older versions of grub customizer that copied a binary file to this location. Newer versions are using a better mechanic.

Try to fix it this way:
sudo rm /etc/grub.d/bin/grubcfg_proxy /etc/grub.d/*_proxy

Then you should be able to launch grub customizer. Some entries may be displayed as removed items. Make sure to restore them before saving the new configuration.