I just had that problem myself this morning. However, in my case it was because a virtual machine ran completely wild and totally trashed the volume that contains my /home. I wound up restoring the entire volume from backup. Very strange issue; I have never seen it happen before. Oh well...such is life, and since my backup procedures are comprehensive, it was just a thing.
Your problem is caused by one of two things: either (a) your /tmp directory is full - and this is a good chance because you said you were fooling around with boot settings - or (2) your KDE settings in your /home directory are screwed up.
My first guess is the problem is with /tmp. You said this is a laptop, which immediately suggests you don't have a lot of RAM. Now, I found out (to my surprise) that there is a setting in Mageia 3 that will cause /tmp to be mounted as a tmpfs in RAM - and if this happens, and you don't have enough RAM, BAD THINGS happen...sort of like the message you are getting...which is a BAD THING.

So, to check this, boot your machine until you get to the log-in screen. After you are at that screen, wait for a few seconds, then type ctrl-alt-F2. This will get you to a console window which will have a login prompt.
At that prompt, enter your user name, then your password. You now will have a command line on which you can enter commands.
At the command line, type the command "df" (without the quotes) and hit enter.
You will get a listing showing where various file systems are mounted. You are looking for /tmp, which might be listed in the right hand column of this listing (and it might not be too). If it is listed in the right-hand column, then follow that row to the left hand column to see what kind of device /tmp is. If the entry in the left hand column is tmpfs, then that is your problem and you need to move /tmp back to your hard drive. If /tmp is not listed or if it is not listed as being a tmpfs, then this is NOT your problem.
Now, if it turns out this is your problem and you need to move /tmp, then you need to know how to do that. Check
https://forums.mageia.org/en/viewtopic.php?f=8&t=5616 for information about that. If that is enough for you, then you are good to go. If not, ask again and someone will help you.
The other possibility is that your KDE configuration is screwed up. What you do in this event depends considerably on how important your configuration is to you. If you are just setting up your system and don't have emails stored on it (particularly if you don't have emails stored on it) then you can just delete the entire .kde4 directory in your home directory and let the system recreate a default KDE configuration when you log in. If you want to save emails it gets a lot trickier.
I won't comment any further until you investigate a bit and can provide enough more information to steer things appropriately, but this will get you going.