The logger has 2x20,000 memory locations (1 temp. 1x humidity). If you save a measurement every hour, that's 20,000 hours = 833 days. However, the battery only lasts for approx. 1 year, data is still saved!
If you save for 15 minutes, there are still 208 days! Everything clear?
Unfortunately, we do not have any similar reports, so if the problem reoccurs, there may be a defect and the item would need to be returned.
Alternatively, we recommend that you post this question as a discussion. Experience shows that more customers answer there than with a single question.
Have a look at Voltcraft. I remember that they have a pretty good manual. By the way: I calibrated the data logger with a calibration thermometer. My copy has a systematic error of -0.2 degrees. I was pleasantly surprised by this low value, especially as the deviation was constant over the tested range.