What do I mean by ”weird problems”? Some websites don’t load fully, you can access network shares but cannot actually open the files, your teamviewer sessions are suddenly disconnected etc. Mind you, lowering MTU on endhosts is not really a good solution because then you need to do it on all your endhosts. Typically, you will lower the MTU on your router, but what if you cannot access your router or the router doesn’t have the option to change the MTU size?
You typically have this problem when on a PPPoE connection, if MTU on the router is set to 1500 instead of 1492. However, it seems that UPC customers have this problem and lowering the MTU on the machines helps, too, because it makes more space for any headers and administrative overhead that is necessary to make protocols work.
Now why does lowering the MTU size help? Because endhosts can fragment a large segment of data better than intermediary routers. Some routers will just do a bad job of fragmenting/assembling the packet. But if the endhost sends packets that are small enough not to be fragmented anywhere on the path from host A to host B, nobody needs to do any fragmentation at all.