I would like to create a profile with a limitation, in User Manager, that will limit the data transfer of a user to a certain quota e.g. 100MB. And then the administrator can refresh the account by resetting the counters. Is this possible?

I have tried but cannot get this simple scheme to work. The user is able to exceed the transfer limit during a session. When logged out, he cannot log back in again, the message is "no valid profile found". In User Manager one can reset counters but still he can't log back in. One has to delete the profile assigned to the user and add it back again.

What is the easiest scheme to limit data transfer to a user and allow the admin to easily refresh it?

thanks for any help.

You can look at using simple queues or mangles.

https://wiki.mikrotik.com/wiki/Manual:Queue

Look here on my website how to limit data transfer.
https://www.wirelessinfo.be/mikrotik-queues/

In dutch but the screenshots lead you to the proper configuration