4shared, MEGA & JDownloader - Troubleshooting
As a rule of thumb...
...
make sure you keep your software up to date. JDownloader gets daily updates and your archiving software (7-zip/WinRar/...) should be up to date in order to avoid compatibility issues with archives.
(4shared & MEGA) Bandwidth limit
4shared:
All free 4shared accounts have a bandwidth cap: 3GB/day and 30GB/month. Hitting the limit will prevent you from downloading files from the browser, or cause an "Account missing" error when using JDownloader. The common workaround is to create several accounts and switch from one to another when needed.

You can register several accounts using different email addresses, then configure JDownloader to use all accounts and switch between them when needed. Simply enter your account credentials in Settings -> Accounts Manager.

JD will automatically use the next account in the list when the bandwidth limit is reached.
MEGA:
May 2018: The bandwidth cap currently seems to be set to 10GB every 6 hours.
(4shared) Only a small number of files are displayed, the rest is hidden (scrolling bar disappeared)
Switch to folder view: change the URL from "http://4shared.com/
folder/xxxxx" to "http://4shared.com/
dir/xxxxx".
(4shared & MEGA)
-One or more files are missing
-Folder is empty
-"The file link that you requested is not valid."
-MEGA asks for a decryption key
Report the download in this section.
(4shared + JDownloader) JD's linkgrabber only grabs 18 files while there are more in the folder
Switch to folder view: chang the URL from "http://4shared.com/folder/xxxxx" to "http://4shared.com/dir/xxxxx".
Select all files in the list. Go to More -> Get links. Copy all links.
(4shared + JDownloader) Files are online and displayed, but the linkgrabber cannot detect them.
In some rare cases, files can be flagged as adult content, and thus will trigger an additional confirmation page - which JD cannot handle.
For instance,
mirror #3435: 12 out of 13 files trigger a "Confirm birth date" page and need to be manually downloaded.