Update 14th of April, 2017 (15th of April, 2017 @ 00:05 AM local time):
It is possible to edit achievements, however, when saving the achievements, it returns the following error:
The edits made on the achievement are not saved and reloading the page returns this error page over and over, unlike the special page Internal Error on the 13th of April.
Note that the name of the error in the image above is called "Call to a member function setEx() on null", while the name of the error shown below is called "Call to a member function getCategory() on null".
__(inserting line is broken, by the way)__
After I noticed that the update on achievements was done, I started translating the titles and change images of the achievements. However, during editing, I encountered a couple of bugs, as shown below.
This is the initial error I get when clicking on 'Save'.
After pressing on F5 to reload the page, a special page named Interne fout (Internal Error) appears with backtrace:
This page is then being displayed every time I reload the page.
Edits on achievements are saved, however, even though these errors are displayed when trying to save edits.
This happened on the Dutch Minecraft Wiki. No idea if this happens on other Wikis, though, and I probably never will know, because it's AFAIK fixed when I edited and saved an achievement a few minutes ago.
Triggering the error
I clicked on the edit button of the achievements that are now listed under the tab 'Bewerkingen maken' (Making Edits). I translated the name of the achievement, its description, the image (for a more Minecrafty style) and translated the name of category. Nothing else was changed as far as I know.
When I clicked on the button to save the edits, the error "Call to a member function getCategory() on null" appeared (shown on the second image of the post). After reloading the page, assuing it was a one-time error, a special page named 'Interne fout' (Internal Error) appeared with backtrace, as shown on the image with a light-blue background. After reloading the page again, I realised it was a bug. The URL stayed the same the whole time (it did not change to '/Speciaal:Interne_fout' or something like that).
I then checked the achievements by opening another tab and found out that the edits were saved somehow. However, it duplicated the category 'Bewerkingen maken' thrice in the drop-down menu for choosing a category. The duplicates were removed when I edited achievements the next day.
The next day I decided to translate the category 'Page Patrolling', assuming the issue was fixed. I did everything as described above, but instead of triggering the getCategory() error, it triggered the error "Call to a member function setEx() on null". The edits were not saved. I then tried to edit an achievement in another category ('Uploading Images') and it triggered the same setEx() error.
I've just finished editing the remaining 12 achievements of the category 'Page Patrolling' (now 'Pagina's controleren') and I have not encountered any issues.
Is it possible to implement a static ID system for achievements, like ID=1 for achievement 1, ID=2 for achievement 2, etc.? This would prevent earning an achievement that was earned before it was edited and put under a new category (likely the cause of this). I noticed this during the process of editing achievements and occassionally checking the recent changes.
I've revoked my earned WikiPoints of the edited achievements, because I have already earned the WikiPoints that came with the achievement before it was edited.