Customization not persistent [resolved]
 

Since my reassignments of shortkeys (Tools - Customize - Keyboard, other customizations there should be affected, too) were not persistent beyond the current session, I searched (help, forum, google) in vain for a solution, (here unwanted) results only concerned the Related Items Pane (I write it this way so as to not create a false positive for searches for that).

Thus, I think it's useful to put the (perhaps obvious, for some) solution Kyle kindly gave me by mail, here into the forum:

Don't have more than one database open (be it in different windows or in different tabs) at the time you do the customization (none at all also works), and then save and close UR. Then only open 2 or more databases in case. No problem anymore this way.

May appear obvious, but I hadn't thought of it. Thank you, Kyle!