Sorry it took me so long to reply but I've been away and not using the Kobo.
Yeah, the files were .acsm. In the meantime I wrote to Kobo help and they said I should write to Adobe and ask for another authorization, since that was probably the problem.
I was wondering, can I copy those files that were mentioned here, from my brother's Kobo to mine and that will solve it or do I have to write Adobe and/or keep removing the DRM from books I buy, to be able to read them in both devices?
Thanks for your input, I was going mad if you guys hadn't mention that DRM issue