I found that downloading the CentOS version and doing rpm2tgz - has worked OK.
Just install the tgz file created using installpkg from the terminal
I have no valid reason for choosing CentOS, just a hunch
Oh don't forget to install the rpm2tgz package, like I usually do the first time.
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The method suggested by laprjns - clean the slapt-src build directory and install from Sbo - appears to work and stays within the Salix/Slackware package ecosystem. That may change if zoom produces more obligatory upgrades, but it's probably the best thing to try first.