Where did the files originate from? Can you post one of the files on Dropbox or another file sharing site.for download. ..or at least post the file's MediaInfo?
I do not think the trial version allows saving files of any type, or maybe just very short length file renders like the Vegas demo. The Fraunhofer MP3 codec is licensed, so MP3s are not supported with the trial version.
I purchased,downloaded,registered,and installed Audio studio 13 directly from Magix and have the same problem. mp3 is not listed in the "open" or "save as" dialogue. when the default all file types is displayed it does not recognize .mp3 files and when trying to open them an error pops indicating that i am trying to open an unsupported file format. Magix tech support has no answer but wanted me to download and run http://support.magix.net/syscheck/syscheck.php?tn=2019122917003766 which is blocked my Mcafee
Just found Fix - open sound forge - upper left click on file then Reset all preferences and cached data - program will restart and look completely different..mp3 are available now.