Actually the missing function is the main problem. The mentioned forum post is related to the J! updates and the core files. It was the same issue, but in another context. Today, those files are up-to-date and do not cause the problem in J!3.2.x anymore. I found the real cause and location right after my last comment. The culprit can be found in the the HELIX II system plugin folder.
It is the following manipulated and most likely out-dated file:
YourJoomla/plugins/system/helix/core/classes/joomla30/bootstrap.php
This file lacks some functions and important changes i.e. deprecated Tab syntax. Additionally, it seems to lack the usual bootstrap loading from J! core JUI folder.
Actually, there are multiple solutions to solve the issue:
A.) Report to JoomShaper, wait for their fix/update (BTW, they are working on a v3)
B.) Compare the original J! bootstrap.php file line by line and update the Helix file. (at least add the missing/new Tab functions... Headsup: this will keep on loading Helix bootstrap JS and CSS)
C.) Simply replace Helix bootstrap.php file completely. (Seems to work! But it will also load J! core bootstrap JS and CSS files)
(PS: On my initial forum post I did not look into the files, because I thought they make use of J! core bootstrap files and do not make any crucial overrides..., unfortunately they load their own BS2 files, which was pretty nice for J25 and as long as it was newer and up-to-date in the early J3.x releases. But Joomla finally updated to BSv2.3.2. I really thought they make use of JUI core files. I also could not get any help from Joomshaper, because I can`t register a regular account!?! I can't get account confirmation. There social login is also a real problem, I am logged in with unknown user credentials! So, i had to wait... or investigate the issue by myself)