Every server has a log. It contains information about what happens on your server. For example, if someone executes a command like changing the gamemode 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. Unfortunately, Minecraft doesn't have a limit on how big a log can get. To protect your and other Aternos servers we're clearing your log if it gets too big. You'll probably see a message like this
Log was cleared, because it was too big.
on the log page. Your log will freeze in that state until you restart your server.
If your log has been cleared you should definitely check 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
/gamerule commandBlockOutput false
in the console.
Please note that you have to restart the server after applying a fix to get the log working again. If your log still gets cleared you probably did not find the cause of the problem.