Within the last few weeks, there have been reports from Delicious Post users where they are sometimes getting the error message, "Error adding link" when trying to post a link to Delicious. I've been experiencing this problem myself when using Delicious Post.
Delicious Post uses the Delicious web API (application programmer interface) to post a web page to one's Delicious account. Any errors from the Delicious side are simply relayed back.
It just so happens that Delicious was recently sold to another company, and these errors started happening shortly thereafter. Coincidence?
There is a workaround to this issue: check if the bookmark title and description contains any special characters, such as the pipe symbol ("|"), em dash ("—") and chevron ("«" and "»"). Try replacing such characters with something safe, such as a comma (","), hyphen ("-"), angle brackets ("<" and ">") -- or if all else fails, the question mark ("?") is always a fallback. Strangely enough, the error occurs when there is a single quote ("'") in either the bookmark title or description, and there isn't an easy substitute for that!
I will be preparing a fix which will require a minor release of Delicious Post. Until then, try the workaround described above. Thanks for your patience.