Contradiction with Matthew 24:44
Matthew 24:44 emphasizes being ready at all times for the Son of Man's coming, whereas Luke 21:28 focuses on looking up for redemption when specific signs begin to occur.
Matthew 24:44: Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh.
Contradiction with Mark 13:32
Mark 13:32 states that no one knows the day or hour of the end, in contrast to Luke 21:28 which implies an awareness of when redemption is near.
Mark 13:32: But of that day and [that] hour knoweth no man, no, not the angels which are in heaven, neither the Son, but the Father.
Contradiction with 1 Thessalonians 5:2
1 Thessalonians 5:2 describes the day of the Lord as coming unexpectedly like a thief in the night, while Luke 21:28 suggests signs indicating when redemption is approaching.
1 Thessalonians 5:2: For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.