I have a recurring issue with Google Inbox when using Google Chrome on my Mac: I keep getting "Server is temporarily unavailable." error messages, and it seems the only way to get them to stop is to close the tab and reopen the site in a new tab.
I've tried doing hard-refreshes before closing the tab, and it does not help. When this is happening, I've verified that other sites work, and that this problem occurs at multiple internet connections (i.e., at work vs. home), so I'm fairly confident that my internet is not the issue.
This problem started within the last week or two (i.e., beginning of December 2016).
Update (12/27): This continues to be a problem. Today I lost a Draft I was working on because I clicked "Send" (after the button finally appeared) and then the "Sending..." message continued to show. After going to Gmail, the last 10 minutes of edits were not saved.
Update 2 (12/27): I just stumbled on a LastPass forum thread suggesting that the LastPass extension is responsible for the problem. I'll follow up with LastPass.
Update 3 (12/28): LastPass replied to me, stating that they were able to reproduce the bug and that their dev team is working to resolve the issue. I'll post an "answer" on here with this resolution if LastPass gets it fixed.