JabRef 5.0-dev--snapshot--2019-03-18--master--52696f04b
Windows 10 10.0 amd64
Java 1.8.0_201

Steps to reproduce the behavior:

1. Open "Groups" panel and try to add a group (or edit a pre-existing one - the problem appears there as well)
2. Give a name (and a description, but that does not change the problem)
3. Hierarchical context is set to default (i. e. "Independent group") and Type is set to default as well (i. e. "Statically group entries by manual assignment")
4. Note, that you cannot create this group -> the "OK" button is greyed out and inactive

Workaround:

5. Set "Type" to "Automatically create groups" - now it is possible to create the group, i. e. the "OK" button is active
6. After using the workaround it is also possible to switch back to the default "Type" for this group, i. e. "Statically group entries by manual assignment" when creating the group

Note, that you have to use the workaround EVERY time you create a new group.