"The following filenames were invalid" Error Message

Hi all. Sorry if this has been covered before, can’t seem to find a relevant thread. Been running an older comicpress for years without issue, decided to relaunch our site and updated everything in the process. After much screwing around, it seems like I’m really close to a working site, but when trying to upload comics I get an error message saying “The following filenames were invalid” followed by the name of the file I’m trying to upload. I’ve tried messing with the date format (I use the standard 2014-09-17 format) in the manager config, nothing I try seems to work. Anyone else have problems with this? My site is https://moonfruitcomics.com, but as of now it’s just a big “Coming Soon” banner that links to a previous version of the site. Can’t start adding content til this issue is fixed – any help would be much appreciated!