Every server has a log. It contains information about what happens on your server. For example, if someone executes a command like changing the game mode or all chat messages are stored in the log. It also contains internal information about your server which helps you to fix your server in case you have problems with it.
A small tutorial on how to check the server log to find and fix problems can be found here.
Depending on how much happens on your server and how well your server works your log may become very very big. If your log exceeds a certain size limit we shut down your server to prevent further problems for other users. In your log you will find a message like this:
[00:21:48] [Aternos System/ERROR]: Server was stopped because the log file is too large.
Try to reduce the amount of logging to avoid this in the future.
To prevent further crashes you should find out what's spamming into your log. In most cases, this is caused by broken plugins & mods or command blocks. The latter can usually be solved by executing the following command in the console.
/gamerule commandBlockOutput false