Created attachment 138395 [details] Screenshot of the refered KStars-Ekos Darks library utility. SUMMARY: In KStars-Ekos Darks library utility,the limit of maximum exposure time is established in 60 seconds, then you can not obtain darks with greater exposure time than that limit.
This is fixed in 3.5.4. You can try nightly packages for 3.5.4 from PPA.