I get random reports from JSM customers getting "You do not have access …" after working in a space for years. The bspace is restricted to added Customers and said persons were added as customers within the space settings.
I was not able to track it to any of the changes. The space setup is the same.
What helped is creating an Atlassian account with the only Customer access to JSM, which should not consume any licenses.
Once invited they were able to access the space no problem.
Any ideas or references to JSM changes?