iPhone 6s with iOS 12.4 not connecting to MacBook Pro mid2015 with macOS 10.14.6
Workaround 1

This is the only reliable workaround for my iPhone.

I came across a similar question. The most up-voted answer lets me charge my phone. However, I consider it rather a workaround than a solution.

Launch "Activity Monitor" > select the usbd process > click the cross icon in the top left corner > select "Force Quit". Sometimes the process has to be repeated once.

Unfortunately, this workaround does not work for my iPad mini 4, which will say Not Charging after quitting usbd.

Workaround 2

This is a weird one... I noticed that sometimes the Trust this Computer? dialogue appeared for a split second. However, it did not appear long enough to actually click  the Trust button (left button). After playing whack-a-mole for a while, I accidentally opened the Calculator App. There I was able to catch the dialogue and then I was asked to enter my Passcode. After repeating this twice it worked even after rebooting the computer. The phone will work randomly for a while.