I had the same problem, I tried with some different nokia chargers and N810 was always rebooting at the nokia white screen.

After that I decided to try with other chargers, and I used one from a Palm device 5,2V an 0.5A (I need to cut the wire and change the connector), and the N810 started to charge.

Conclusion: You can try with a bit more of voltage, maybe it isn't the perfect solution but it is a solution.