"low disk space" whereas there is space on other volumes
 

I get a warning stating there is "low disk space". But if I look to "On this computer" I see 3 disks: one "computer", a second reserved system and a third 91 GB volume (27.5 GB/90.6 GB available /dev/sda2). What can I do to increase the usable space? The output od fdisk -l is:
Thanks for your advice!

Since you are posting on a Linux forum, I'm guessing you get this error when you are logged into whatever Linux you are using, correct? Which Linux might that be? When you open a terminal and run: df -h you should see output telling you how much space is used on various mounted partitions and how much space is available. Run that and post the output. You also have two windows partitions and I'm not sure what the reference to sda2 means, that is a windows filesystem on the partition. What exactly is it that you want to do, expand which partition? shrink which partition.

For something more usable run these.

Like suggested
will show you what is having what, so you can locate where it is talking about. If you want to increase your space you have to move your partitons, then murge the free space into the other one.

I am running on Ubuntu 18.04.
The infos requested:
other informations:
Thank you for your help!

You appear to have a Legacy install with your Linux system on a Logical partition and your windows partitions as primary. A 25GB root filesystem partition should be more than enough for a home user so you must be installing a lot of new software or using the partition to store data which might go elsewhere. It's technically possible to revise/shrink your partition (sda2) and then use part of that for you logical partition but that can be hazardous as you would need to move boot files. Simplest solution, get another hard drive. If you want to try moving/resizing, post the output of the command:

Lower Case Letter L in the command. This will output more details such as sectors for partitions.

From post #1...

So the "volumes" mentioned in the title are partitions - and the "free" space is as mentioned simply unused space in the Windows NTFS file system. It is not unallocated space you can easily assign to your Linux partition.
It is possible to shrink the /dev/sda2 and add it to /dev/sda5 (your Linux partition) but is a little fiddly. Not technically difficult, but must be done in stages.
here is an article on using gparted - not exactly your situation but close. In your case you need to (in this order, and clicking "Apply" each time) - Sorta like a sliding block game:
- shrink /dev/sda2
- drag the left boundary of /dev/sda3 to occupy that space
- slide /dev/sda5 all the way to the left
- drag the right boundary of /dev/sda5 all the way to the right to occupy the free space.

All done. Note that the bootloader issue mentioned above shouldn't be a problem - grub uses filesystem services to locate the boot code and can handle being moved like this.

resizing
 

Yes, Yancek I would like to resize my partitions.
The output of was given in my first post.
I'd appreciate some guidance, thanks!

A brief outline in the post above by syg00 then go to the link posted there with detailed explanation of the process. You might use the windows Disk Management tool to shrink the larger partition, then reboot windows to check before following the other steps.

gparted limitations?
 

I cannot resize sda2 to 58.73 G and get 25.62 G unallocated space, when I click "apply" I get the following error: gparted_details.txt

The message above from GParted indicates you are trying to shrink the windows partition (sda2) down to a size of 58.73GB. Problem with that is your df output from post #5 above shows that partition is 85GB is size with 59GB used. All systems need a certain amount of free space and you are trying to shrink it to a size below which the data existing. You need to select a smaller size. I would also suggest you use windows Disk Management although GParted should work.

remove Windows
 

I would like to remove completely the Windows partition sda2 and use the freed space to increase the Linux partition sda5, how can I do that? Cheers!

Not easy as sda2 is physically in FRONT of sda5.
The normal procedure would be: backup sda5 onto other (external?) storage.
Now remove sda5, sda3 and sda2 (but remember where sda2 STARTED and sda3 ENDED).
Create a new sda2 with starting value of the old sda2 and end value of sda3;
this partition now will have all of the space of the old sda2 and sda3 combined (sda5 was INSIDE sda3).
Reboot the system to let the kernel see the new partition layout.
Create the file system of your choice in /dev/sda2 (for ext2/3/4 use mke2fs)
Finally mount the new partition and restore your backup'ed data into that fs.

PS: when you're doing that, you may want to create a swap partition too, then the end of sda2 must be smaller then sda3 was, so space is left for a swap partition. It should be of type "Linux swap" and either sda3 or sda4 can be used for it.

You might be better off to take this opportunity to segregate user data from the root filesystem, by deleting sda1 and sda2, and creating a new sda1 using all the freed space to use as /home/. To help us help you determine whether that makes good sense, show us output from:This would be a much simpler and massively less dangerous process than moving/merging/resizing your current root partition.