Some of the top directories are a a bit unexpected.
1.1G /opt/CPDiffReportServer
1.5G /opt/CPrt-R81.20
2.2G /opt/CPda
2.7G /opt/CPsuite-R81.20
3.1G /opt/CPInstLog
5.0G /opt/CPuepm-R81.20
5.4G /opt/CPshrd-R81.20
I would go over /opt/CPInstLog and see if you got ancient stuff in there.
Having said that the error could apply to ither the root or the /var/log partition.
Step 1 is to run `df -h` to understand which partition is filling up.
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