2022-02-09, 16:00
Another possible source of problems:
If there have been too many errors with requests, Google may set your daily quota to zero. This may happen, for example, if you've been trying to use the old keys too many times. It may be so that the quotas may reset after a while (I'd presume they do) once the "developer" doesn't send erroneous queries anymore (i.e. in this case, fixes the keys) but I'm not sure how long it takes nor if it will ever actually happen; I didn't try waiting. It might take 24 hours, it might take a month (the quota page does say "Daily quotas reset at midnight Pacific Time (PT)" - so it would make sense that's when the error quota also resets).
So case may be, you need to create another project, and this is actually what I did. To check if you have this problem, check out the quotas in the YouTube Data API section in the developers console Web UI. All quotas should have a sensible non-zero value; and IIRC it will also tell you error quota has been reached (don't remember the exact wording, didn't take screenshots; the error quota itself is not displayed on the page, don't know what the actual value for it is). If a quota (Queries per day) has been set to zero, wait a while (until midnight PT) or create a new project if that doesn't help - or in case your impatient, like I was.
If there have been too many errors with requests, Google may set your daily quota to zero. This may happen, for example, if you've been trying to use the old keys too many times. It may be so that the quotas may reset after a while (I'd presume they do) once the "developer" doesn't send erroneous queries anymore (i.e. in this case, fixes the keys) but I'm not sure how long it takes nor if it will ever actually happen; I didn't try waiting. It might take 24 hours, it might take a month (the quota page does say "Daily quotas reset at midnight Pacific Time (PT)" - so it would make sense that's when the error quota also resets).
So case may be, you need to create another project, and this is actually what I did. To check if you have this problem, check out the quotas in the YouTube Data API section in the developers console Web UI. All quotas should have a sensible non-zero value; and IIRC it will also tell you error quota has been reached (don't remember the exact wording, didn't take screenshots; the error quota itself is not displayed on the page, don't know what the actual value for it is). If a quota (Queries per day) has been set to zero, wait a while (until midnight PT) or create a new project if that doesn't help - or in case your impatient, like I was.