Unexpected errors in update.php during the custom plugin installation

Hello guys,
I’ve spent a couple og hours in finding the answer but still don’t succeed.

we’re developing the theme for our client. He will sell that on the Themeforest.

He tests the theme on localhost and today he faced the issue of following warnings https://prntscr.com/6yywax

* We’re completely not able to reproduce that on our machines in our office (we’re in different countries with our client).
* He says warnings occur with our theme only, other themes work fine. So, the firewall, proxy and other infrastructure options don’t seem the reason
* He says warning occur on both his local machines
* He uses 4.2.1, but the issue occur on the 4.1.1 also. We had 4.1.1, updated to 4.2.1 and still don’t see anything wrong
* He usee xamp, php 5.5. We tried to reproduce on php 5.5 also, but no result

Could you at least give a piece of advice in what direction should we investigate? How can we reproduce that issue on our machines?

Any help will be great, we’re completely frustrated by this issue.

thanks in advance.