The phone is a Hero H2000 dual sim with Android v2.2.1, MTK Platform
I get an Invalid IMEI error after a factory reset. How can I fix this?
There are several possibilities. The first starts with checking the "Related" section to the right of this page, which gives you some good candidates which already contain answers/solutions, e.g.:
Next, there are some video tutorials on Youtube on this topic, e.g.:
My short Google Search also found some forum threads providing hints and solutions, e.g.:
I couldn't verify any of the mentioned solutions myself (luckily, I've never been affected by this problem) -- but I hope at least one of them will prove helpful to you.