No need to format drive or update RouterOS. The key part is, cache administrator's name must have to be the same as Store name. And a reboot is necessary before re-enabling web-proxy.
Pls. follow these steps:
1. Disable proxy & clear cache. No need to reset HTML.
2. Go to: System>>Stores, click "+" sign, Type a new Store name (this must have to be the same as cache administrator's name). Select type as "web-proxy", check "Active". This will make old web-proxy store "backup". Remove the old one. Click apply and ok.
3. Here come's the most important part, that is, reboot your router before re-enabling web proxy.
4. Go to System>>Stores, here you'll see the new web-proxy store as "backup". Make sure there is no other "web-proxy" type store.
5. Go to IP>>Web Proxy. Type cache administrator's name, same as store name you have given. Check "cache on disk", and set Max. cache size as "unlimited". Now check web-proxy "Enabled", apply and ok.
6. Go to System>>Stores, you'll now see the "web-proxy" drive as "active".
7. Visit some web sites, click "cache contents" in web proxy settings to see it's working or not.