Minecraft – My texture pack won’t show up in client

Maybe, Minecraft is blocked by Windows Firewall. Go to Control Panel, and search "Windows Firewall" in the search bar. Click "allow a program or feature through Windows Firewall," and make sure Java is allowed.

The user MorkPork had a similar issue in another question. He had the same error while trying to use a url contained in a string in java. Another user m-szalik, responded with this answer:

You need to encode your parameter's values before concatenating them to a URL...

In another question, good asked which characters are not valid in a url. User Gumbo responded saying that the characters []@!$&'()*+,;= are all invalid. As m-szalik said, you must encode them. In the url you gave the only invalid character was =. I already encoded the equals symbol for you using a Url encoding site. The result was %3D. Once we replace the equals sign with the encoded equals sign, the result is:

https://www.dropbox.com/s/4nbkkrfmxkw6n82/DefaultHDv1_7.zip?dl%3D1
If you click that URL it works just the same as the old url.

Note: If that url does not work either you can also try encoding the entire url, which would result in http%3A%2F%2Fwww.dropbox.com%2Fs%2F4nbkkrfmxkw6n82%2FDefaultHDv1_7.zip%3Fdl%3D1, which is also a working Url.

I hope this helped.

Tip:
In a url a backslash is not enough to make it valid. This is because the backslash is an invalid character itself (However, forward slashes / are valid)

Sources:
Which Characters Make a Url Invalid? asked by good, answered by Gumbo.

Java.net.MalformedURLException: no protocol on URL based on a string modified with URLEncoder asked by MorkPork, answered by m-szalik.

Url-encode-decode.com. scripted and maintained by Dan from DansTools.com.