Please explain what you do and what you get.
Any error in ProjeQtOr log file ?
Any error in browser console ?

Hi, I've the same problem.
I just upgraded from Version 8 to 9.0.6.

Description : The attached file (pdf in this case) can be previewed through the pdf viewer, but when I click on the arrow in order to download the file, nothing happens
And also in order to reply to your 2 questions :
* No error in the ProjeQtor log file : It's not detected as an error... means nothing appears in the log file
* No error in the browser console (chrome) : In the attached screenshot, you can see on the bottom left that Projeqtor find the correct path of the file 

Thanks in advance for your suport

Very strange you can see preview, but not download for same attachment.
In fact both should work or both should fail, not one work the other fail.

But I'm quite certain it is migration issue.
In mos cases the issue is due to migration with new copy for new instance, without recopying local files (attachments, documents,....)

Check location of attachment files : location as defined on global parameters and corresponding directory on system.
Check for existing sub-folder containing attachment_n° where n° is id of attachment. It should contain the file.
Check rights on folder and on file (PHP user must have read rights)
 

Thanks @babynus for your reply.
I find it very strange as well.
Regarding your checklist please find below in red  the result :

Check location of attachment files : location as defined on global parameters and corresponding directory on system.  location  in the server and declared on global parameters are the same 
Check for existing sub-folder containing attachment_n° where n° is id of attachment. It should contain the file. #65 in the GUI =  Folder named attachment_65 and the file is inside 
Check rights on folder and on file (PHP user must have read rights) In order to be sure, I apply a chmod 777 for folders and files 

But if it helps, I tried  the folowing 
- click right when I select the arrow Download 
- open the link in a new tab
.... And you know what ? the download starts and the file is saved in the local PC

Try and clean your browser cache 😉

Good idea... I tested it but no, it does'nt work
I disabled as well my antivirus and my antimalware.... It doesn't work
In addition, the buton "Delete"  works correctly and does the job 

Which browser are you using ?
Notice that depending on options, download may not be very "visual".
For instance, on Firefox, if you select to always download files, without prompting, when you click on download, you just have small blinking icon of blue arrow on top of browser, but file is correctly download.
 

So check your do<nload folder after clicking on download icon.

Hello ! 
I'm using Google Chrome and regarding your suggestion, I tried with Microsoft Edge.
For both of them it doesn't work and nothing in the dowload folder (test with the internal feature of each browser and check in the windows folder)
 

@rmiroff
Hello ! You find the way with your issue ? If yes, please share 😉

Hi,
it is actually very common problem with these browsers, we experienced as well; following two links helped:
https://www.tenforums.com/browsers-email/30826-cant-download-edge.html
https://www.easeus.com/resource/google-chrome-not-downloading-files.html#3
hope this helps!
Note: no such issues on linux clients

To confirm it is a browser issue; just test on Demo : https://demo.projeqtor.org.
Log in as manager and go to ticket #143.
If you cannot download the attachment on this ticket, it is a browser issue, that you'll have to fix yourself (see mkavurcic's post)
If you can download it, its is a server issue on your instance.

Thanks mkavurcic and babynus for the follow-up : 
* Test with  demo.projeqtor.org = Confirmed, the download doesn't start
* Test with mkavurcic's tips = Even I did what they advice, it doesn't work (Test on 3 different PCs with Edge, Firefox and Chrome)
So strange... 

that sucks;
if your platform is w10, check this out:
https://windowsreport.com/cant-download-files-windows/
 

one more tip:
to completely exclude server side problem try downloading attachment from Linux client