@Plany Kao I have managed to find a workaround for me hopefully it also helps you.
The issue seems to be that the users affected do not have general access to the app. In our case they are in the AD group which grants global app user access but it does not seem to be pulling through for them.
I have added the individuals as "app users" into global big gantt configuration and this resolved the issue.
we have the same problem here in our company, some users (all from the same group) can access BigGantt and some, like me, can't.
We already contacted our administrator, and he said that this is also most likely due to a bug.
Please fix this asap, bugs like this are super annoying and frankly speaking quite audacious since we're paying lots of money every year for this tool and now can't use it at all. The project management department has basically come to a standstill because of this!