Server Crashes and Multiple Timelines

Hello!

Recently I have been having some odd issues with my server. It is a regular vanilla survival server 1.12.2. The issue is that when people join the server it sometimes crashes entirely. To make things worse, when it crashes it creates what I refer to as an "Alternate Timeline" where, upon rejoining, all players get jumped back in time, undoing everything they've done, to the point where the last player before the crashing player joined. Another issue that has resulted from this is when we restart the server, it will say online through the website but it will not be online for any of us. The final issue I'm having is that, upon restarting, some people can't join at all until we actually stop it all the way and restart it again while others can join just fine.

All these issues have led to a few woes for my players and it would be very helpful if we knew how to fix them.

Thanks! -Lavivrus

No. It changes.

I haven't tried it with a new world. Yes, I think it has only happened with the third or higher player that has joined. I have a suspicion that it's because the server doesn't have enough memory but I'm not sure. Do you have an idea what could be causing this or how we could fix it?

Command blocks are enabled but we aren't using any due to it being fully legit survival. I am also unsure what you mean by trying a new world. Do you mean creating a new world and then switching back to our old one or just trashing the old one in general?