You would need to read the manual to get the real answer but I'll take a swing at it. Frame apropos to Layer 2 communications usually refers to framing alignment errors. On the datalink layer, of which PPP is an example, information is sent in frames. We refer to frames instead of packets at this layer to distinguish them from L3 Network Layer packets.
When no frames are present on the L2 media, the line is idle. This means when the frame starts the receiving serial controller must have a way of getting clued to the fact that a frame is about to begin and what the first real byte is. For this reason most L2's serial protocols use the 7E flag, or 01111110, at the start and end of the frame. This is a bit sequence that can't appear in the rest of the frame data due to a process called bit-stuffing, which I'm not going into here.
A frame allignment error means that the serial controller didn't pick up on the flag, so couldn't read the frame. You can't really call it a data error, because it might not have been a real data error, it can't tell cos it couldn't delineate the frame.
As to why you are getting these, thats more difficult. Could be anything from a noisy line, although I would have expected more real errors as well, to a slipping clock, to just poor electronics on your router.
You need to look at what the timescale is for these errors appearing , if they all happened in the last 10 seconds, and continue then you have a real problem. If they don't continue, forget it you just had a line hit, if they clock up at a few every week, then also forget it, it won't be causing you any problems.
As I said, read the manual or contact the manufacturer as this might possibly mean something else.