Forum OpenACS Development: Warning about Tcllib 1.21 with OpenACS
This is a warning to wait a little bit before using tcllib 1.21 (released May 2021) inside OpenACS. Between tcllib 1.20 and 1.21 several changes were introduced in tcllib using now reflected channels , such as e.g. in the mime package. Before that, reflected channels were not used by the tcllib packages used by OpenACS.
Unfortunately, it turned out that there is a bug in all the released version of Tcl in connection with thread cleanup and reflected channels, causing a (hard) crash. This bug is present in at least Tcl 8.6.9 - 8.6.13, Tcl 8.7* and Tcl9. Fortunately, this bug is now fixed  in all the variants of Tcl, but it will take some time until new releases containing these fixes will come out.
For the time being, it is recommended to stay with tcllib 1.20.
All the best