Adobe Premiere Error: I have no idea what this means

[lgrumet]

Hey there i have been reading these forums for a while, so its actually not my first visit

i have tried to find previous answers to my question but unfortunately i was not succesful
the problem is the following:
i have to different videos. the first video is from a normal videocamera, which i importet with the windows wizard to my pc.
the second video is a screen capturing video, this means I used a software to film what exactly happens on the screen.

alright. the first video is a .wmv file, the second a .avi

now to the problem:
i have put both videos together in premiere, next to each other.
afterwards, i tried to export the video (i used the adobe media encoder). the settings do not seem to matter, since I get the same error also when I use any different kinds of settings. So during the exporting process, approximately at 17%, I get the following error:
Error while exporting, no video frame could have been given back.
(this is translated from the german version...but the sentence doesnt make much sense to me in german either.
Fehler beim Exportieren: Es konnte kein Videoframe zurückgegeben werden)

i have this whole scenario 5 times. (merging 2 videos together). It worked without any problems 3 times. but with two videos, I get this stupid error and I have no clue why. I am sure I worked always in the same way, so there are no technical differences between these 5 projects. All have the same source (camera and software).

Could anyone please help me?

[lgrumet]

by the way, i just seem to have found the correct error message in english: "Failure to return Frame"