OK, I found the following in the log files. Kstars/Ekos seems to run out of file handles:
[2021-11-13T17:06:21.665 CET INFO ][ org.kde.kstars.ekos.capture] - "Capturing 1.000-second image..." [2021-11-13T17:06:21.798 CET DEBG ][ org.kde.kstars.indi] - ZWO CCD ASI294MC Pro : "[DEBUG] StartExposure->setexp : 1.000s " [2021-11-13T17:06:25.223 CET DEBG ][ org.kde.kstars.indi] - ZWO CCD ASI294MC Pro : "[DEBUG] Uploading file. Ext: fits, Size: 23397120, sendImage? Yes, saveImage? No " [2021-11-13T17:06:25.648 CET DEBG ][ org.kde.kstars.indi] - ZWO CCD ASI294MC Pro : "[DEBUG] BLOB transfer took 0.310453 seconds " [2021-11-13T17:06:25.649 CET DEBG ][ org.kde.kstars.indi] - ZWO CCD ASI294MC Pro : "[DEBUG] Upload complete " [2021-11-13T17:06:25.915 CET DEBG ][ org.kde.kstars.indi] - Image received. Mode: "Normal" Size: 23397120 [2021-11-13T17:06:26.060 CET INFO ][ org.kde.kstars.indi] - "FITS" file saved to "/home/astroberry/Ekos/Light/test_Light_473.fits" [2021-11-13T17:06:26.060 CET DEBG ][ org.kde.kstars.fits] - Reading file buffer ( "22.3 MiB" ) [2021-11-13T17:06:28.304 CET INFO ][ org.kde.kstars.ekos.capture] - "Download Time: 2.65 s, New Download Time Estimate: 2.65 s." [2021-11-13T17:06:28.319 CET INFO ][ org.kde.kstars.ekos.capture] - "Received image 473 out of 500." [2021-11-13T17:06:28.334 CET INFO ][ org.kde.kstars.ekos.capture] - "Captured /home/astroberry/Ekos/Light/test_Light_473.fits" [2021-11-13T17:06:29.908 CET DEBG ][ org.kde.kstars.fits] - FITHistogram: JMIndex 1 [2021-11-13T17:06:30.183 CET DEBG ][ default] - WARNING: Phonon::createPath: Cannot connect Phonon::MediaObject ( no objectName ) to Phonon::AudioOutput ( no objectName ). [2021-11-13T17:06:33.738 CET DEBG ][ org.kde.kstars.indi] - ZWO CCD ASI294MC Pro : "[DEBUG] Uploading file. Ext: fits, Size: 23397120, sendImage? Yes, saveImage? No " [2021-11-13T17:06:34.166 CET DEBG ][ org.kde.kstars.indi] - ZWO CCD ASI294MC Pro : "[DEBUG] BLOB transfer took 0.313525 seconds " [2021-11-13T17:06:34.167 CET DEBG ][ org.kde.kstars.indi] - ZWO CCD ASI294MC Pro : "[DEBUG] Upload complete " [2021-11-13T17:06:34.415 CET DEBG ][ org.kde.kstars.indi] - Image received. Mode: "Normal" Size: 23397120 [2021-11-13T17:06:34.549 CET INFO ][ org.kde.kstars.indi] - "FITS" file saved to "/home/astroberry/Ekos/Light/test_Light_473.fits" [2021-11-13T17:06:34.551 CET DEBG ][ org.kde.kstars.fits] - Reading file buffer ( "22.3 MiB" ) [2021-11-13T17:06:36.773 CET INFO ][ org.kde.kstars.ekos.capture] - "Download Time: 2.60 s, New Download Time Estimate: 2.65 s." [2021-11-13T17:06:36.787 CET INFO ][ org.kde.kstars.ekos.capture] - "Received image 474 out of 500." [2021-11-13T17:06:36.801 CET INFO ][ org.kde.kstars.ekos.capture] - "Captured /home/astroberry/Ekos/Light/test_Light_473.fits" [2021-11-13T17:06:36.840 CET WARN ][ default] - QProcessPrivate::createPipe: Cannot create pipe 0x977e3bc: Too many open files [2021-11-13T17:06:38.420 CET DEBG ][ org.kde.kstars.fits] - FITHistogram: JMIndex 1