While writing this forum entry text, I kept experimenting with a few further ideas to solve the problem, and by chance I could fix it. Nevertheless, this may help others, so I am still posting this:
On vanilla linux systems, I have just installed flexlm with a valid (new) license file on one machine (license server).
On another machine, I have installed the full compiler package (icc, ifort, mkl, etc., Version 16.0.3.210 Build 20160415), choosing activation via the just installed flexlm on the other machine. Upon query by the installation script, I entered the port number (28518) and machine name, and after a while the script told me
"Activation completed successfully"
Assuming that all is fine, after the installation finished (and after running compilervars.sh), I tried to use "ifort", but kept getting this message:
Error: A license for Comp-FL is not available (-76,61026,2).
License file(s) used were (in this order):
... ... ...(some lines deleted).... .... ....
ifort: error #10052: could not checkout FLEXlm license
Using telnet, I have verified that from the other machine I can reach the license server machine on port 28518.
And, as suggested elsewhere in this forum, I have generated a license checkout log by setting the environment variable INTEL_LMD_DEBUG (letting its contents point to a filename of my choice); sadly, however, the long output generated there did not really help me (although presumably it does help the experts).
In the above error message text, at the location "some lines deleted", several directories are listed where the compiler searches for a license file. Coming from earlier installations w/o license servers, I mistakenly assumed that with flexlm in place and after having made successful contact with this flexlm during compiler installation/activation, it is o.k. that in those locations there really is no license file -- because the compiler should contact the license server, and there of course a license file is present (I copied it there). HOWEVER, getting desperate, I simply copied my license file (the same one I copied to the license server) also to one of those locations, and *bingo* things instantly worked.
What I still do not understand is why a local license file is still necessary, after successful activation of the compiler suite during installation, and (accepting for the moment that this is needed) why the compiler installation script (install.sh) failed to provide for that or at least instruct me to do such a copy manually.
(By the way, I also wonder why this version of install.sh does not remind me to source compilervars.sh; all the previous versions always did that, at the end of the installation.)
So, again, maybe the above helps some other users,
and just out of interest I still would appreciate an answer to those questions at the end.
Bernd