Check how your Runtime minification settings are configured.
Try setting it like this:
"RuntimeMinification": { "CacheBuster": "AppDomain" }
Check if the user under which the server is running has write permissions.
Also, check if the folder exists on the server:
Smidge/Cache/Your-Computer-Name/*
If the folder exists and you’re using a CDN (for example, Cloudflare), try clearing the CDN cache.