Hi, i am running a iMac 5K with Mojave (10.14.3) and have the latest Express 6.12. (which autoupdated after I installed a old 6.11 version for a try)
I needed to exchange a device Vivosport because of a RMA replacement.
But as soon as I try to add the device and click on the "login" button I used to get a screen to enter userid and password.
now I only get e screen "500 server error"
Syncronising the already known "installed" devices worked, even updating maps worked (until I did the next step)
I followed the tips about complete deinstall, removing the plist (prefs, app. support, cache) emptying trash and rebooting
no success, now I have no devices and can not add any new ones :-(
trying to do the same with a WIN10 machine on the same network worked without problems.