How to reproduce:
1. Create a macOS account with a full stop at the end of the "Account name" (i.e. "jacob." or "emily.")
2. Log into that account
3. Open Unity
4. Try to create a new project

Expected result: Project is created.
Actual result: Project is not created due to "Project path is invalid - make sure you are not using invalid characters".

Reproducible with 2017.3.0a2, 2017.2.0b5, 2017.1.0p2, 2017.1.0f3, 5.6.3f1, 5.6.2p4

Note: While using full stops at the end of the folder name is not supported by Windows shell, macOS allows such naming strategy, so creating a project should be possible. Opening a project seems to be working.