sorry, didnt get an email notification about an answer ..
so this is what I did:
I stopped the container, deleted the 2 files and then started it again.
same problem - this is the log file (please read from bottom to top)
---Something went wrong, can't download Thunderbird, putting container in sleep mode---
2021-03-01 20:44:37 stdout ---Thunderbird not installed, installing---
2021-03-01 20:44:37 stdout ---Version Check---
2021-03-01 20:44:36 stdout ---Starting...---
2021-03-01 20:44:36 stdout Nothing to do, noVNC compression set to default
2021-03-01 20:44:36 stdout Nothing to do, noVNC qaulity set to default
2021-03-01 20:44:36 stdout Nothing to do, noVNC resizing set to default
2021-03-01 20:44:36 stdout ---Checking configuration for noVNC---
2021-03-01 20:44:36 stdout ---No optional script found, continuing---
2021-03-01 20:44:36 stdout ---Checking for optional scripts---
2021-03-01 20:44:36 stdout ---Setting umask to 000---
2021-03-01 20:44:36 stdout usermod: no changes
2021-03-01 20:44:36 stdout ---Checking if GID: 100 matches user---
2021-03-01 20:44:36 stdout usermod: no changes
2021-03-01 20:44:36 stdout ---Checking if UID: 99 matches user---
2021-03-01 20:43:21 stdout Session terminated, killing shell... ...killed.