The Router operates more than 2 months and now after reboot i lost license key

I see you have upgraded recently to a new license key but what happens when you click 'update key', as it suggests?

(I suspect it will not make any difference.)

After downgrade to 3.30 key is back
very strange ....

http://wiki.mikrotik.com/wiki/Upgrading ... nse_issues

Click the update license button now while you are on 3.x. The license should update to the new format, then upgrade to 4.x

fewi - if you examine the screen shot, the RB while on 3.x already has the new 8 digit xxxx-xxxx licence key.

I would say that an upgrade to 4.x will now work OK. But if it does fail again - contact support with a supout file attached.