That isn't something Firefox controls you need to contact Proton VPN support about connections issues.

That's an unusual message. I didn't find a single match in Google.

Do you get the error only on sites where you need to log in, or do you see that for all sites?

Any difference in a private window (Ctrl+Shift+P)?

Are you allowing cookies as some VPNs can block cookies ?

• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/introducing-total-cookie-protection-standard-mode

What other extensions do you have ?

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems