Apple photos displays exclamation mark instead on HEIC images
MacBook Air with macOS 10.11 (El Capitan) Apple photos app displays preview thumbnails for all photos, but some photos do not open in full view when double-clicked. Instead of a photo, a grey exclamation mark is displayed. On a closer look it became clear that only HEIC format files were affected, JPGs were opening just fine.
Finding Apple Mail attachment edited and saved by MS Word
A user attached a Word document to an Apple Mail email from his desktop, then opened it from Apple Mail with MS Word, made multiple changes and saved it by clicking File > Save. He then closed the document and was surprised to find that the changes he made were not present in the attached document, nor in the original file saved on the desktop.
Change MS Office 365 for Mac license
Microsoft Office 365 for Mac is licensed by a valid Office 365 license, but we need to change it to a different user license.
Mac OS - This Connection is Not Private
Web browsers (Chrome and Safari) give warnings "This connection is not private" on certain HTTPS websites. After a closer look, it appeared that all affected websites were using Let's Encrypt certificates.
The computer in question was a 2011 MacBook Air running El Capitan Mac OS. The issue was caused by DST Root CA X3 root certificate expiration in September 2021. As explained by Let's Encrypt, this certificate has been replaced by ISRG Root X1 which is now trusted by pretty much all up to date operating systems. The problem is that El Capitan (and some other no longer supported older OS's) never received this root certificate therefore do not trust it.