Broken SSL cert caused by router?

[Mike <mike-nanog () tiedyenetworks com>]

Hi,

    I have a very odd problem.

    We've recently gotten a 'real' ssl certificate from godaddy to 
cover our domain (*.domain.com) and have installed it in several places 
where needed for email (imap/starttls and etc) and web. This works 
great,  seems ok according to various online TLS certificate checkers, 
and I get the green lock when testing using my own browsers and such.

    I have a customer however that uses our web mail system now secured 
with ssl. I myself and many others use it and get the green lock. But, 
whenever any station at the customer tries using it, they get a broken 
lock and 'your connection is not private'. The actual error displayed 
below is 'cert_authority_invalid' and it's "Go Daddy Secure Certificate 
Authority - G2". And it gets worse - whenever I go to the location and 
use my own laptop, the very one that 'works' when at my office, I ALSO 
get the error. AND EVEN WORSE - when I connect to my cell phone provided 
hotspot, the error goes away!

    As weird as this all sounds, I got it nailed down to one device - 
they have a Cisco/Meraki MX64W as their internet gateway - and when I 
remove that device from the chain and go 'straight' out to the internet, 
suddenly, the certificate problem goes away entirely.

    How is this possible? Can anyone comment on these devices and tell 
me what might be going on here?

Mike-