"Add link" attempt for a Menu yields 500 error I have tested with fresh installs of Commerce Kickstart down to version 2.6, the error happened. I even disabled all modules related to the Menu module or with menu in the name, no luck. I also disabled and then enabled the Menu module, no luck. However, I tested version 1.19 and it did no yield this error.
Change MYSQL max_allow_packet_size.

Incase you are on MAMP you can add --max_allowed_packet=32M to /Applications/mamp/bin/startMysql.sh and restart MAMP.