Strange one this. In Joomla 3.7.5 with k2 2.8.0 when in Admin I can create and save a K2 Item so long as there is no hyperlink in the content. As soon as I add a link and try to save I get a 404 Error - Cannot Find Category.

I have searched for a solution, but most threads are very dated and no firm solution is put forward.

Any ideas please?

I cannot duplicate this on a similar install. What editor are you using? I am using JCE to insert the link into introtext and everything works as expected

Hi, thanks for picking up on this. I have just spent the intervening time trying to work out what the problem is.

The first thing I tried was changing editors, but that didn't work.

After spending time excluding possible causes I now think that there were some unauthorised characters in the URL I wanted in the link, which failed the Save test. I have worked around it by editing the URL in the link and the item is now saved and published.

Out of interest, is it K2 that sets a list of excluded characters? If so where can I see these as I would like to know that I have found the culprit.

Check the options at the top right of the k2 componentm, it may be in there

I can't find it there.

Hello Mick,
Check your error log for any possible PHP errors.

Hi Krikor, yes did that and none relevant to this error. I am pretty sure that the URL I had for the link was the problem and it contained prohibited characters. It would just be good to know where the exclusion list is.

Sorry, its Parameters not Options, and i was thinking about the box full of SEF URL character replacements on one of the pages.
There is a Content Cleanup section in the parameters that i think if all off by default but you may have turned on. Probably not your problem

Thanks, I checked in Parameters and found the box full of characters, but it says it is for earlier versions of Joomla.

Content cleanup settings are Off.

Can you send me an example of your URLs/aliases?
FYI: stackoverflow.com/questions/1547899/which-characters-make-a-url-invalid