When in checkbox mode, space would normally select the checkboxes. But
after reading a post here, i changed the “destination toggle” function to the Space bar, because it is really handy for changing l/r display. Now selecting files in checkbox mode won´t work anymore, although Ctrl-Space is possible, but somewhat unpractical. Can the selector key be changed, maybe to Alt-Space?
I don't think it can be.
It's not just checkbox mode; space selects files in all modes.
Using space to toggles sides is probably a bad choice of key if you want to use the keyboard for selection as well. It's a bit like overriding one of the cursor keys. 
BTW, Ctrl-Tab will switch between the two sides by default, in case anyone is wondering if there's a default/standard key for it.
(Tab and Shift-Tab will switch back and forth as well, but they'll also cycle through things like the location field rather than just between the two halves of a dual file display window like Ctrl-Tab.)
Hmm, i´d like to keep the space key for changing sides 
I know about Ctrl-Tab, of course, but for me it´s too off, since i prefer to control most actions in the right hand area around right Ctrl / arrows buttons, like when using a mouse. That´s why i also used a second “toggle sides” shortcut in that area. That way i can copy/move/back/fore/up/cycle tabs/toggle sides/move item one higher/add items to clipboard or clear it/go to various folders, etc. - controlling all those things from that small range, which is very cool & damn quick. Space key is more universal & easier to hit than most other combos.
Since i only find this bothersome in checkbox mode, i think i can live with Ctrl-Space as selector keys, because for all other selection tasks i didn´t use Space anyway.
I can select either by arrow / shift arrow keys for selecting contiguous portions in details mode (including Pos1 & end keys for quick marking) or, if files have to be selected randomly & not blockwise, Ctrl-Arrows (for navigating to the separate files) & then selecting by Space key, which still works then, at least in Details/Power modes.
Anyway, since currently i use literally dozens of shortcuts, it sometimes isn´t easy to avoid side effects.
Now i assigned Shift-Up to perform a "select first", which works nicely.