Hard to solve browser-captchas: What to do?

Some Browser Captchas may be hard to solve sometimes.
While this article is mainly about Googles reCaptchaV2, the basics of it will also fit on other browser captchas.

Symptoms - how do I recognize hard to solve captchas?:
Instead of a single "I'm not a robot" click captcha like this:

... you may encounter multiple challenges in a row to solve e.g. "Click on all traffic lights", "Click on all cars" or similar:

There is no way to determine the factors which e.g. Google is using to decide which captchas you get 100% but there are some things known to "make these captchas easier to solve".

(Again, this article is mostly about the widespread reCaptchaV2.)

1. Use Google Chrome --> Google prefers users who use their own browser
You can keep using the browser of your choice and use Chrome only for JD! See HERE!

2. Disable your proxy/VPN if you are using one.
If you are using a proxy/VPN in JDownloader, consider using the same via browser (and the opposide way too).

3. Make sure you are logged in into a Google account in the browser you are using to solve browser captchas.
A "real" Google Account which is not only used for captcha solving is better than one which you've just created for the purpose of solving captcha.

4. If you are in a network where a large amount of people are sharing one external IP address e.g. company- or university network, this can also cause hard to solve captchas.
In this case, a private VPN might even be your solution but first you should change your network and test if you get better captchas e.g. via mobile network or with your home IP address (as long as it is not used by a lot of people).

5. If you are solving a lot of reCaptchaV2 captchas for other users via external portals e.g. 9kw.eu to earn credits, make sure that you are using a separate Browser or browser profile, separate Google account and a separate IP than you are using when JD is running.
If you don't, you may get hard to solve captchas simply because you're solving "too many" per hour/day and it may take some time until Google "likes" you again and the captchas get easier.

FAQ

It is asking me e.g. to select all airplanes but there are no airplanes, is this a bug??

This is perfectly normal and again: Those captchas are generated by Google.
In this case, just click on the "Skip" button.