Could it be the name of the solution itself? The name of the solution is Europe 2014. Would it matter if that were the same as that of other mods on the Steam Workshop?
EDIT: Nope, I changed the name of the entire solution in addition to the package file, didn't make a difference. I think it may be an issue with mod size so I'll probably have to go through a tedious effort of splitting my mod into pieces.
EDIT 2: I managed to upload all of the audio and art components of my mod (31,038 KB worth) as a separate mod, which leaves me with a 286 KB CIV5MOD file to upload. However, I still keep getting invalid parameter, which leaves me very confused. Apparently apostrophes aren't special punctuation, because the name of my audio-visual components mod, nguyenforthewin's Europe 2014 Components, uploaded without a hitch, but this mod, nguyenforthewin's Europe 2014, will not. I am very confused at this point and am not sure if I should continue splitting the mod into smaller pieces because I don't know if file size is the problem. The remaining file to be published has Lua, SQL, XML, and the scenario map.
EDIT 3: Only more frustration. I separated out the Lua and SQL files, so now the XML files and the map are sitting by themselves. And yet, even though I managed to publish the Lua and SQL files separately, the XML files and map will not publish. This final package failing to upload boils down to 216 KB... I'm not exactly sure what to do to make this work. I'm going to incorporate the map into the Lua and SQL to see if that publishes and then see if there is any issue with the XML.
EDIT 4: I sliced the map off into my second components mod, and it published, leaving behind a 169 KB CIV5MOD file with only XML... and the remaining XML still won't publish. I'm guessing that maybe I can publish if I divvy up the XML contents, but I'm not totally sure. There are small xml items in my Lua and SQL so I don't think it's like XML is being automatically rejected. I may just have too much XML in one place. In any case, I think this is a problem I'm going to solve later.
EDIT 5: As a last measure before opting to break up the XML, I changed the mod ID. Still couldn't publish. I'll have to reorganize the XML tomorrow.