EvolutionM is a licenced software and therefore we have a set amount of users that can be signed in at any given time.
This issue normally arises when management reports are due as an influx of people are trying to use the software at the same time and there is a peak of users above the license count.
If you get this message you just have to try again later when some other users have logged off. You will be able to gain access eventually as the licenses shuffle around.