There are sporadic reports of this issue, but I haven't been able to reproduce it. My best guess is that it's due to trying to use the custom build before Galaxy has finished creating it. To ensure that the custom build is ready for use, make sure to wait for Galaxy to show the number of chromosomes rather than '?' on the custom builds page.
It also sometimes happens when I create and share a Trackster visualization. I can access the visualization without problem whereas the users I shared it with can't access it because of the mentioned error or they get a message like "Can't load dbkey".
"Can't load dbkey" is a different issue than the server error. If you can provide datasets and/or steps to reproduce either issue, I'm confident that they could be fixed. Thanks, J.