I installed the trial version of Confluence Questions and immediately received the following error:
"The currently installed version of this add-on is not compatible with your Confluence instance. A paid update is available for this add-on."
what can we do to evaluate this product?
If you are in Confluence server you have to be running these versions: Confluence Server 5.3 - 5.5.4 as in https://marketplace.atlassian.com/plugins/com.atlassian.confluence.plugins.confluence-questions.
Feel free to verify the version compatibility and give it another try again.
I know, however there was no other option. It was directly from within Confluence (not the marketplace) so selecting a version wasn't an option and now it says I need to do a paid upgrade. I didn't select an older version...this was the version that was added when I did it through Confluence directly and it's not until it's installed that the version is displayed.
That sounds like you trialled it on an older version, removed it, upgraded Confluence then tried to get another trial. Confluence won't have installed an old version that was incompatible with the current install, so that's the only way I can imagine you can get into this state.
You could try removing the plugin completely, shut down confluence, clear the plugin caches thoroughly (See https://confluence.atlassian.com/display/CONFKB/How+to+manually+remove+malfunctioning+add-ons)
Then re-install from the UPM, which will get the right version. Problem is that I suspect it might still detect your old trial data and do exactly the same - demand that you pay and upgrade.
It's not yet a year old, so it was a bit more recent. Thing is, they shouldn't have trialled it in a production system. That's the awful way to do a trial - it should have been done in a test system which you can tear down and rebuild from your production data.
To continue the trial, you've got two basic options - strip the data out of your database (potentially dangerous and quite hard work), or fire up a temporary trial of Confluence + Questions on a test system and decide if you want the addon. If you do, then you'll pay for it and get your old data upgraded. If you don't, then your old data doesn't matter.
I'm afraid I don't know the database well enough. I'd start with shutting down Confluence and going through the "clean out plugins" routine, but after that, you'll need to drop all the data that the plugin has inserted, and I don't know where that data is.
I'd be strongly tempted to do the wipe of the test instance, but I do understand that may not be easy to arrange.
Having said that, the other option is a second test instance for it. You can have many of them if you've got developer licences, so it's just down to the effort involved in building a new one - I suspect that's less work than working out exactly what to hack out of the database safely.