HTBFileopen DLL problem solved!
One of the recent changes made to HTBasic 2021 was changing the name of the executable from HTBwin.exe to HTBasic.exe (to make it more obviously the main executable). However, it turns out that some of the DLLs in the library, including HTBFileopen.dll, reference HTBwin.exe.
So, a new release will be issued next week that changes the installation name back to HTBwin.exe. But as a temporary workaround, you can verify that renaming HTBasic.exe in the install folder back to HTBwin.exe fixes this problem. Obviously, it was not anticipated that simply changing the name of the main executable could have this affect, but at least it’s a simple fix.