[Users] Migrating emails and emails folders from "Mozilla Thunderbird" to "Claws Mail"

[Lmhelp]

Hello and thank you for reading my post.

I'm migrating from "Mozilla Thunderbird" to "Claws Mail" and I'd like
to migrate my data (emails and emails folders).

In "Mozilla Thunderbird", I presently have "n" email addresses.
For each email address, I have several folders and subfolders
where I put my emails. For example, for one email address, here
is the kind of structure that I have: 
//----------------------------------
an_email_address at an_isp.ext
  - Inbox
  - Drafts
  - Sent
  - Trash
  - a_dir_1
      - a_sub_dir_1_1
          - an_email_1_1_1
          - an_email_1_1_2         
          ...
          - an_email_1_1_k1
      - a_sub_dir_1_2
      ...
      - a_sub_dir_1_n1
  - a_dir_2
      - a_sub_dir_2_1
      - a_sub_dir_2_2
      ...
      - a_sub_dir_2_n2
  ...
  - a_dir_N
      ...
//----------------------------------

I would like to retrieve all my emails and the directory structure
above in "Claws Mail".

In "Thunderbird":
1) I downloaded the "ImportExportTools" addon.

2) I chose "Tools" -> "ImportExportTools" -> "Options" -> "Export
directories" and checked "Export folders as MBOX file" to a directory
"D".

3) I then selected the email address "an_email_address at an_isp.ext" and
chose: "Tools" -> "ImportExportTools" -> "Export all the folders (with
structure)"

Now, in "Claws Mail":
1) I selected "an_email_address at an_isp.ext".
2) I created a "a_dir_1" folder.
3) I chose "Import mbox file..."
  and for the "Mbox file" parameter, I chose "D/a_dir_1" (which was
exported previously).

It ends up with an error:
"Mbox import has failed"

Can you tell me what I should do to recover my data into "Claws Mail"?
Thank you and best regards.
--
Léa Massiot