According to Accu-Chek's customer care website, an E5 error message on their Compact Plus model could mean that the blood droplet was applied too soon or the testing strip was bent. It could also mean that the measurement window needs cleaning or that there is moisture within the meter.
If retesting with a new strip does not correct the problem, the unit may need to be cleaned. It can be cleaned by first turning the unit off then opening the drum door. Once open, the unit may be wiped down with water, a 10 percent bleach or 70 percent ethyl solution or rubbing alcohol and allowed to dry, according to the manufacturer.