PowerPoint 2021 crashes when saving .ppsm files containing ActiveX controls

Your cross-computer reproduction and the clear break between build 2602 and later builds strongly indicate a PowerPoint regression rather than a damaged profile or installation. Since the same presentation saves as .pptm, use that format as the workaround and keep an untouched copy of any original .ppsm file. Submit the smallest reproducible sample through File > Feedback with diagnostic data enabled, and add the exact builds, crash time, and Windows Reliability Monitor event to the existing support case. If .ppsm output is business-critical and Office 2021 is installed with Click-to-Run, an administrator can temporarily revert to the known-good build using the Office Deployment Tool. That choice should be controlled centrally because an older build also omits later security and reliability fixes. Disable updates only for the shortest necessary period, test each new build with the sample, and re-enable updates as soon as Microsoft confirms or you verify a corrected build.