I looked again in log files you posted.
I finaly found on oldest one (I must admit I first looked only at the 3 most recent ones) :

2015-11-16 11:17:00.908 ** ERROR ** ERROR **
2015-11-16 11:17:00.908 ** ERROR ** on file '/var/www/html/projeqtor/tool/saveAttachment.php' at line (244)
2015-11-16 11:17:00.908 ** ERROR ***** cause = mkdir(): Permission denied

It is clear : php user don't have write rights on /projeqtor/files folder to create the expected thumbs folder.
So either give write rights to php user on /projeqtor/files
or create /projeqtor/files/thumbs and give write rights to it for php user.

Please see the following screenshots for permission for files.

I believe the permissions are okay?

Is it normal that the thumbs folder is completely empty?

Please see the following screenshots for permission for files.
I believe the permissions are okay?

Yes, they seem correct.

Is it normal that the thumbs folder is completely empty?

Not it is not, it should at least contains thumbs for attachements as a picture.
And the issue sure comes from this point.
New must understand why generation of thumb fails...

Are you sure you have nothing in ProjeQtOr log files ?
And maybe in Apache error log file ?

Could you also chack prerequisites as defined : http://www.projeqtor.org/en/product-en/technical-en/64-setup-en/412-product-prerequisites-en

Please see attached apache log file.

There are definitely a lot of errors.

There is an erroe "Call to undefined function imagecreatefromjpeg() in var/www/html/projeqtor/tool/file.php"

This could be why the thumb is not being created.

GD library is missing !

That did it!

Thank you!