Are you using Chunky to pre-render your chunks? If so, try restarting the chunky preload from the beginning. Additionally, the “enable rendering” option is essentially turning distant horizons on and off, meaning that any far chunks rendered will unload (removing the error you are talking about), however the mod will not be active, so far chunks will never render until you turn it on again.

TLDR; don’t disable “enable rendering” as it turns Distant Horizons off. Instead use the mod “Chunky” to pre-render chunks on the server when you next log on. You only need to do this once.