This is since upgrading to 5.0.0.5
Which Linux distrib do you use? For the test, could you rename your LO directory profile? (see https://wiki.documentfoundation.org/UserProfile#GNU.2FLinux)
We cannot triage issues when people only write one sentence fragment as bug description. - What is your locale? (displayed in Help > About) - How did you upgrade? Did you use distro-provided packages or you downloaded manually from libreoffice.org? - Thesauri are external packages, e.g. the "mythes-fr" package. Do you have that installed?
Since there were no details given for several months, I assume the reporter managed to get the thesaurus working.