There is a field called RewardText. Somehow it has too many characters. Likely it is because you had extra spaces in it or a paragraph spacing and the HTML tag it entered caused it to exceed the maximum length of the field.
I had a similar error come up last night and it took a bit to track it down. It was due to a paragraph spacing being translated to HTML and exceeding the limits.
What I would do in your case is to go through any field that has anything to do with a Reward, click in the box and see what the numbers at the upper right of the box say. You will likely find one that is something like (102/100) or similar. Just pare down the text a bit so it is under the maximum character length for the field.
You might also want to open the .STORYARC file for the mission using Notepad or another text editor and use the Find feature of that text editor to search for RewardText. This should let you see what the line of text is to find it easier using the MA Editor. On a default PC installation of the game, I believe the path to that file is C:\Program Files\City of Heroes\Missions and the file is named however you saved it. For example, if you named it FREEM when you saved it, the name of the file is FREEM.storyarc in that folder. If you haven't saved it with a name yet, there will be a autosave.storyarc file if you have enabled the auto save feature. If you haven't yet enabled the Auto Save feature, I'd recommend doing so as soon as possible. And remember, save often, save again, save, save, save. It's a hard lesson to learn, but well worth it.
If I knew exactly which fields were named what, I'd be able to give a better starting point. Maybe in a month or two I'll get it all figured out, if I keep messing with the MA. Maybe someone that knows the fields better will answer Soon™ and give you a better idea of exactly where to look.
Good luck.
If the game spit out 20 dollar bills people would complain that they weren't sequentially numbered. If they were sequentially numbered people would complain that they weren't random enough.
Black Pebble is my new hero.
RewardText is what you set to put text in the chat window when a goal is accomplished. I forget what it's labeled in the UI; it might depend on the goal type.
I found this by opening my .storyarc file in Wordpad and searching for "RewardText".
And for a while things were cold,
They were scared down in their holes
The forest that once was green
Was colored black by those killing machines
So I wanted to test mission now that the orange error box went grey. Click save and test, and I get this pop up window.
"The field RewardText exceeds the text limit."
What gives?