Please indicate your Ubuntu distribution version and architecture (32bit / 64bit) as well as the exact package you have installed (file name).

Did you install any non-standard / downloaded fonts?

Ubuntu 20.10 64bit
qcad-3.25.2.4-pro-linux-x86_64.run

I have many fonts installed. I did try to reproduce with CourierCAD.

[~/.local/share/fonts]
(0): ls
adobe-fonts/                     Montserrat-SemiBold.otf
otf-hermit-2.0/                  Montserrat-ThinItalic.otf
SourceCodePro/                   Montserrat-Thin.otf
Unknown\ Vendor/                 Ubuntu\ Mono\ Bold\ Italic\ Nerd\ Font\ Complete\ Mono.ttf
Montserrat-BlackItalic.otf       Ubuntu\ Mono\ Bold\ Italic\ Nerd\ Font\ Complete\ Mono\ Windows\ Compatible.ttf
Montserrat-Black.otf             Ubuntu\ Mono\ Bold\ Italic\ Nerd\ Font\ Complete.ttf
Montserrat-BoldItalic.otf        Ubuntu\ Mono\ Bold\ Italic\ Nerd\ Font\ Complete\ Windows\ Compatible.ttf
Montserrat-Bold.otf              Ubuntu\ Mono\ Bold\ Nerd\ Font\ Complete\ Mono.ttf
Montserrat-ExtraBoldItalic.otf   Ubuntu\ Mono\ Bold\ Nerd\ Font\ Complete\ Mono\ Windows\ Compatible.ttf
Montserrat-ExtraBold.otf         Ubuntu\ Mono\ Bold\ Nerd\ Font\ Complete.ttf
Montserrat-ExtraLightItalic.otf  Ubuntu\ Mono\ Bold\ Nerd\ Font\ Complete\ Windows\ Compatible.ttf
Montserrat-ExtraLight.otf        Ubuntu\ Mono\ Italic\ Nerd\ Font\ Complete\ Mono.ttf
Montserrat-Italic.otf            Ubuntu\ Mono\ Italic\ Nerd\ Font\ Complete\ Mono\ Windows\ Compatible.ttf
Montserrat-LightItalic.otf       Ubuntu\ Mono\ Italic\ Nerd\ Font\ Complete.ttf
Montserrat-Light.otf             Ubuntu\ Mono\ Italic\ Nerd\ Font\ Complete\ Windows\ Compatible.ttf
Montserrat-MediumItalic.otf      Ubuntu\ Mono\ Nerd\ Font\ Complete\ Mono.ttf
Montserrat-Medium.otf            Ubuntu\ Mono\ Nerd\ Font\ Complete\ Mono\ Windows\ Compatible.ttf
Montserrat-Regular.otf           Ubuntu\ Mono\ Nerd\ Font\ Complete.ttf
Montserrat-SemiBoldItalic.otf    Ubuntu\ Mono\ Nerd\ Font\ Complete\ Windows\ Compatible.ttf

Thanks. Unfortunately, this is not reproducible here. My best guess would be that a font causes the problem.

If you can try running QCAD under gdb (if you are comfortable with that) and post a backtrace I might be able to offer more help:

LD_LIBRARY_PATH=. gdb ./qcad-bin
run
# produce crash here...
bt

Thank you.
I don't know gdb but simple steps I can do. When the program froze I typed bt. See lines beneath.
While typing this comment the program was still going. But then I noticed it responded again.
That was with a test case with courier CAD.
Then I tried with my own drawing with Monteserrat. The program froze and ran again for 5 min or so on full cpu. After that the height input works as expected. But when I restart the program the whole thing starts again.

Starting program: /home/cccplex/opt/qcad-3.25.2.4-pro-linux-x86_64/qcad-bin 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
QCAD version  3.25.2.4
[New Thread 0x7fffee7aa640 (LWP 14081)]
[New Thread 0x7fffed942640 (LWP 14082)]
[New Thread 0x7fffe7264640 (LWP 14084)]
12:35:37: Debug:    trying to load theme stylesheet:  themes/Default/stylesheet.css
12:35:37: Debug:    trying to load theme stylesheet:  themes/Default/stylesheet_linux.css
12:35:38: Debug:    CadToolBar.initStyle
Warning:  action not found: "VisibleInCadToolBar"
Warning:  action not found: "VisibleInMatrixPanel"
Warning:  action not found: "VisibleInMenu"
Warning:  action not found: "VisibleInToolBar"
[New Thread 0x7fff4d0d7640 (LWP 14089)]
[New Thread 0x7fff488d6640 (LWP 14090)]
[New Thread 0x7fff4c8d6640 (LWP 14091)]
[New Thread 0x7fff4c0d5640 (LWP 14092)]
[New Thread 0x7fff4b8d4640 (LWP 14093)]
[New Thread 0x7fff4b0d3640 (LWP 14094)]
[New Thread 0x7fff4a8d2640 (LWP 14095)]
[New Thread 0x7fff4a0d1640 (LWP 14096)]
[New Thread 0x7fff498d0640 (LWP 14097)]
[New Thread 0x7fff2bfff640 (LWP 14098)]
[New Thread 0x7fff2b7fe640 (LWP 14099)]
[New Thread 0x7fff2affd640 (LWP 14100)]
bt

[Thread 0x7fff2b7fe640 (LWP 14099) exited]
[Thread 0x7fff2affd640 (LWP 14100) exited]
[Thread 0x7fff2bfff640 (LWP 14098) exited]
[Thread 0x7fff498d0640 (LWP 14097) exited]
[Thread 0x7fff4a8d2640 (LWP 14095) exited]
[Thread 0x7fff4a0d1640 (LWP 14096) exited]
[Thread 0x7fff4b8d4640 (LWP 14093) exited]
[Thread 0x7fff4c0d5640 (LWP 14092) exited]
[Thread 0x7fff4b0d3640 (LWP 14094) exited]
[Thread 0x7fff4c8d6640 (LWP 14091) exited]
[Thread 0x7fff488d6640 (LWP 14090) exited]
[Thread 0x7fff4d0d7640 (LWP 14089) exited]

I tried some other versions.

In this version the freeze starts when starting the text tool.
qcad-3.25.0-pro-linux-x86_64
qcad-3.25.1-pro-linux-x86_64

These versions all have the a fore mentioned problem.
qcad-3.25.2-pro-linux-x86_64
qcad-3.25.2-pro-linux-qt5.14-x86_64
qcad-3.25.2.4-pro-linux-x86_64

This version works normally.
qcad-3.24.3-pro-linux-x86_64 works ok.

This is the result after I opened the text tool and it worked for minutes.
Apparently it has a problem with the numbers and it show nan.

And when I type 1 in the height input it outputs nan.

So I tested again with snapshot 25.2.7
The way I reproduce now is as follows:

1. Open the attached 'text-scaling.dxf
2. edit the text (courierCAD height 2)
3. change the main font height to 200
4. enter
5. edit the text again (all is selected automatically)
6. click the local height input
7. press backspace twice (the result should be 2)
8. before going to 2 the freeze happens (this takes a few minutes)
9. enter
10. edit the text again (the selection lines are huge and no text is visible in the tool window (see screenshot)