Redshift flickers in Xubuntu 21.04

[quiversky]

Describe the bug A clear and concise description of what the bug is.

1. After booting up, the screen will flicker very fast - light and dark, then go to a slow 4 second cycle - 2 seconds light, 2 seconds dark
2. Sometimes, two lightbulbs will appear in the tray and the screen will flicker quickly

• [ ] I have checked the FAQ and my issue is not mention there.

To Reproduce Steps to reproduce the behaviour:

1. Install redshift

redshift.conf.txt

3. Scroll down to '....'
4. See error

Expected behavior A clear and concise description of what you expected to happen.

Error output/logs/screenshots If applicable, add screenshots to help explain your problem.

Software versions (please complete the following information):

• OS: [e.g. Linux/Windows/macOS]
• Redshift version: [e.g. 1.11, run redshift -V in a terminal]
• Distribution: [e.g. Ubuntu]
• Redshift installed from: [e.g. "using apt-get", "built from source"]

[sicherist]

I have the same problem in Kubuntu 21.04

[sicherist]

Probably the same as #810

[vrossum]

Probably two instances running. Check with ps uax and kill one, or killall redshift-gtk and start one up again.

To the developer: redshift should have one instance max. Some programmes have that.

[mbeko]

I have the same issue after updating to 21.04. Since Redshift hasn't had a new release in the recent years, I assume it's because of the update to Python 3.9.

I'll see if I can fix that properly. Restricting to one instance should be implemented for such a program in general, but we should also fix the root cause here.