In previous versions it was possible to export the WATO folder permissions of each user to NagVis. This was only used for maps that are named exactly like the WATO folder name. If you don't have such NagVis maps you are not affected by this change.
This feature has never been documented officially,
It was disabled by default and could be enabled with the global setting "Export WATO folder permissions".
When having a large amount of users (1000+) and/or a bigger folder hierarchy while having the global setting enabled this lead to a huge performance impact when editing several things in WATO (users, roles, groups, ...) and during activation of changes.
In case you really used this feature, you now need to configure the permissions manually e.g. by defining contact groups and giving them permissions on the NagVis maps (via WATO).