I'm tracking the SystemData since May more or less every day. The graph is like a saw tooth: SysData grows about 6GB per day. Then the system behaves strange. Everything is slow and laggy and eventually the SysDatat goes back to normal (like 10GB) and the cycle begins from start. I have no clue what you can do about this. But deleting your browser cache, it is not.

Uninstalled Apple Music and all my songs. Unfortunately, it did not help. But at the end it's just annoying. When the filesystem runs full, something happens and SystemData goes back to a reasonable usage and the cycle repeats.

Doesn’t help. Like all the other tipps as deleting browser history and so on. Updating to iOS 15.6 solved for me the issue.

What do you think is the first thing a user does, when he/she is affected by this error? Maybe it helped in your case. But if you read the complete thread, you'll see: it's not that simple.

The last update to 15.6 seemed to fix this issue for me. But I think they just changed the display. SysInfo shows 2 GB of free space only, whereas in iOS it show 13 GB of free space. When the filesystem runs full, the phone becomes very laggy and unresponsive. Which it is doing right now. Hope iOS 16 brings some relief.