acro package only printing long forms (if forced, can print short forms)

Hello dear Stackexchangers,
I’m a little stupefied by following problem. I’m using the acro package. When I call the acronyms with "ac", it only prints the long forms. I have prepared a MWE:

documentclass[final, 12pt, USenglish, a4paper, titlepage, BCOR=15mm, DIV=14, headsepline=false, footsepline=false, twoside, openany, cleardoublepage=empty,    listof=totoc, index=totoc, bibliography=totoc, numbers=noenddot, parskip=full, captions=tableheading]{scrbook}
usepackage[utf8]{inputenc}
usepackage{xspace}
usepackage{acro, longtable} 
acsetup{list/template=longtable}
begin{document}
frontmatter

DeclareAcronym{slta}{short = {SLTA}, long = {superlongtestacronym}}

mainmatter

ac{slta}ac{slta}ac{slta}ac{slta}
acs{slta}acsp{slta}

backmatter
end{document}

The output is:

superlongtestacronymsuperlongtestacronymsuperlongtestacronymsuperlongtestacronym
SLTASLTAs

Any idea what could have gone wrong here? I’d rather have the standard behaviour of first printing the long form with the short one in captions, and the short one afterwards, unless specified otherwise.
Thanks so much!

documentclass{scrbook}
usepackage{acro, longtable} 
acsetup{list/template=longtable}

DeclareAcronym{slta}{short = {SLTA}, long = {superlongtestacronym}}
begin{document}

ac{slta} ac{slta} ac{slta} ac{slta}

acs{slta} acsp{slta}


end{document}