First you are using, or at least appear to be trying to use, two different terms that don't go together. You keep adding "er" to the end of message, which makes me feel like you are talking about Messenger, the Facebook product. Later you say that you uninstalled and installed Handcent, which is an SMS app, and some may refer to texts as a message or messages. No, "er". Not having clarification on which app you are actually using and which is sending you the error message makes giving advice difficult.
Since you mention Handcent I will give you a few things to look at and/try for regular SMS issues of texts not being sent.
>Do you have a lot of text messages, I mean a lot? I've seen failure of SMS sending, being received and delays in either due to a high volume of messages being kept on the device. Usually when I've seen such failures the user has thousands of messages, and while it may seem that uninstalling the app and installing it again will give the user a fresh start, the truth is those messages exist in the native app on the device. As you witnessed I'm sure all of the messages just re-populated when the app was restored. This is because the app only manages the messages for the user and the native app actually stores and handles the messages. Try reducing the number of messages you have by actually deleting some. By some I mean hundreds if not thousands. It may not be easy to do from Handcent and may be easier from the native app its self.
>Is your device running low on storage? This one I can't explain quite as well but being low on storage can affect SMS. Again I don't really know why, I've just seen it happen, so if you are running low and can free up some space on the device this could help.
>Do you restart the device regularly? This one is easier to explain and goes back to one of the things you tried already. Restarting your device helps close apps running either in the foreground or background, it also helps to clear caches for many apps. So either restarting your device regularly and/or getting an app like Ccleaner from the Play Store to help you clear them can help. In the past Facebook, Instagram and Messenger were the big ones to be sure to hit when clearing caches. Why would a cache from another app be affecting your SMS, I can't really answer that either. My guess is that even caches from other apps can consume resources that a device needs. (Just a side note, I would probably just use CCleaner and uninstall it. It's just a tool.)
If the notification takes you to something other than Handcent the issue is not with Handcent. I've had Handcent and if the message fails it will show you the message in the conversation that failed. Usually with a red triangle and exclamation mark, so if this takes you to Messenger but gives you little or no information, start checking your Messenger conversations for failures. Perhaps the file was too big, the person blocked you or unfriended you. I really can't give you more than that without more information. An easy way to determine if you have the right app is to go into your device settings, Settings>Apps>Messenger or Handcent and force stop the app. Whichever one causes the notification to go away is the one responsible.
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