Yes, now my bootloader is locked and also OEM unlocking is disabled.
To summarize:
After initial flashing of iodeOS 7.6 (over 7.5) I had these problems with the cameras and speakers and also the extreme booting periods (factory reset and redownloading and reflashing iodeOS 7.6 could not solve the issues).
Then I installed the stock Fairphone OS and did NOT lock the bootloader afterwards. Just booted in stock OS and checked if everything is working again (it did).
After checking that bootloader is still unlocked, I flashed iodeOS 7.6 and everything worked. The OS runs smoothly.
And then I locked the bootloader and disabled OEM unlocking in the Dev Settings. And still everything works fine. 
For me, though, the detour via the stock OS was necessary to sort of reinitialize the system or something like that…