Invalid system activity (sysstat) file on Ubuntu server

You are trying to use a file which is not a system activity file, or whose format is no longer compatible with that of files created by current version of sar

Just remove the files:

rm /var/log/sa/sa??

I have found that this happens when adding/removing (v)CPUs. Doing that probably adds/removes columns in the data file format or something.

Removing the "invalid" saXX files and simply running sar caused it to regenerate and start collecting data again.

I have found this issue when I change my instance type in EC2 to higher one which modified the CPU too. So, because of this there is a issue with sar.

Inside /var/log/sysstat/ there are two types of file named with saXX stores data. So you just move those saXX files to some folder and then run

sudo service sysstat restart

It will fix it :)