1. are you certain it is the twilight forest. there are twilight biomes in the worlds (including overworld), too.
2. it happens on world generation, so if you reset it, it needs to generate everything again.
If you are only a small group, tell you players not to run around too much in the twilight forest. Whenever a chunk is being requested which hasn't be loaded (generated) before it will generate it and has the chance of crashing the server with this issue. So as long as you stay in regions where you were before (the part being already generated) there is no problem.