Strange "Faults" - Did you know that...
Several of these 'incidents' were not jDownloads related 'challenges' but due to things outside jDownloads but they may appear to be a jD problem!
As other such problems are found then they will be added to this article.
Example plugin activated
Site seems slow
Most of the Downloads were unchanged. So it seemed a possible solution was that increasing the cache size might help. But this did not solve the problem. And in any case increased cache size does not help first time or occaisional users.
The site had high security and was using IP blocking with Akeeba Tools. Now that meant extensive IP checks were being done on every page load. As it was the Downloads that really needed the protection then the IP blocking was switched to the jDownloads option 'Activate IP Blocking' in the Options tab Security. This IP blocking only operates when attempting to download a file.
This also speeds up the general switching between jDownloads views say from a Downloads Listing to a Downloads Detail or Summary view.
So speed problem solved and security maintained.
Error 500 after updating
Where does jDownloads select font Colour?
Green Bar on Video Images and/or Colour Distortion
Usually this is with a Firefox browser.
See https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems
for various causes/solutions.

After turning off hardware acceleration the green band problem was fixed and colours were back to normal.

The DP Calendar Extension and the jD Content plugin
However sometimes this goes wrong!
Investigation determined that problems only occured when the content plugin was added for a Download in the first line of the Description.
So a line like below where the content plugin, which has been slightly highlighted for clarity, is in the first line of the DP Calandar description:
"Here you may download additional information

will cause 'challenges' with symptons such as other modules 'vanishing' and no further calandar events being shown.
However if the DP Calendar description is like:
"You will find some additional information
by clicking on the download

All then works OK
NB the DP Calendar people are aware of this 'challenge' so it may have been fixed.
jDownload Category List View seemed to only appear once!
This sort of symptom is consistent with having caching active. It could be the Joomla cache, or perhaps the Browser cache. But clearing those caches may not solve the problem!
Some web modems, particularly those working with a satellite, also have a significant cache. So it may be necessary to reboot your web modem in order to clear its cache.
Using Regular Labs Components Anywhere
The challenge here was that this worked AOK in jD 3.2. It also worked in jD 3.9 when SEF was turned off but failed whe SEF was turned ON.
The solution was a careful reading of the Components Anywhere documentation as things seemed to have changed with SEF in Joomla! 3.9.
In particular check out the Component Anywhere tutorial sections 'Using Menu Item...' and 'Component Parameters'.
The link was formerly
{component index.php?option=com_jdownloads&view=category&catid=XXX} where XXX is the relevant catid
Now to be independant of the SEF status, the link style requires the menu item as well so that the structure is shown below where YYY is the id of the relevant menu item.
{component index.php?option=com_jdownloads&view=category&catid=XX&Itemid=YYY}
jD Views appear twice!
The component causing the problem was Ordasoft - Joomla CCK - Website Builder https://ordasoft.com/cck-content-construction-kit-for-joomla.html which is a high quality component when using its Quickstart templates.
The problem is, at Nov 2020, being resolved.
