Thanks for this hint: there was indeed a problem with a directory ~/.cache that has been created without my knowledge: that filesystem has only 2MB free space, so at one point there was probably a problem with "no space left on device". ~/.cache points now to a bigger partition, so there is no more such problem.
How could I debug this problem please?
maybe quicklisp doesn't have enough space for the libraries? or for it's metadata? afaik (not sure) quicklisp maitains it's own cache for something. Try to put the quicklisp on the bigger partition too.
Also, if you don't want to depend on Quicklisp, you may consider creating a bundle with it (quicklisp has a functionality for that).
Best regards, Daniel