I don't know why all the errors I had were so unhelpful and all the documentation I found didn't say this, but the issue was this:

My custom repository block in pom.xml needed this change:

<url>file://SHAREDSERVER/custom/repo</url>

should have been

<url>file:\\SHAREDSERVER\custom\repo</url>

Note the direction of \ vs. /! This is the direction of the slashes as used in Windows file explorer! (This is for a repository that is on a shared network location.)