Status bar could not find cached time string image. Rendering in-process

This bug/debug note is present in iOS SDK bundled with Xcode 9.3+

Update: not fixed in Xcode 9.4.1 (9F2000)
Update: not fixed in Xcode 10 (10A255)
Update: not fixed in Xcode 11 betas

I think you can safely ignore it. New versions of iOS/macOS sometimes contain informational debugging messages, and they generally get removed at the next release.