I can find many sources stating, that you can declare the interval of schedules like: 1d, 6h, 30d, ...

Offical source:
https://wiki.mikrotik.com/wiki/Manual:System/Scheduler

But it seems like you can enter the interval in webfig and android app just in the format: 00:00:00
If you enter a interval like 6h you can't save, a simple alert-message appears saying: "Invalid value in Interval". Good job - classic error-message by a (bad) programmer.

And even for that format I can only guess that it is: hh:mm:ss

BUT on the terminal - thats the funny part now - I am able to create new schedules with e.g. "interval=6h".

That's kind of inconsistent. ("kind of" is a pure understatement here)

Are there ANY offical docs for the scheduler?

"Bad" programmer?

Incomplete requirements or missed in QA; but actual the programmer did a fine job here. Input is properly validated and saved according to some spec.

Be a "good" user and report your findings to support via mail (support@) or https://help.mikrotik.com/servicedesk/ :-)

Input is properly validated

Obviusly it is not properly validated. As the format "<number><unit>" should be possible.
Actually I am refering to the non-helpful message: "invalid value". That's the cheap & lazy way. "Hey user - wrong input. Over and out."

Gonna report - valid point.

Just found this post. Actually, to schedule 1 day interval, you type the following in the interval field: "1d 00:00:00".