Hi Frey,
What OS i sthis problem occurring on?
Open up the Windows System Event Log and see if there is a message about SEP's minifilter not being able to load. A reboot usually resolves that.
Another cause can be if the defintions downloaded by the SEP client are corrupted and not able to be used.
I recommend downloading and running the SEP Support Tool. That is very good at automatically detecting errors and issuing warnings. Follow its recommendations and the problem will likely be resolved.
Please keep the forum community up-to-date with your progress!
Thanks and best regards,
Mick