The problem is solved. After ensuring a proper structure of the glossary to be uploaded, I was able to upload several thousand term pairs in one file via the Deepl API.
Thanks for the support!

Karl, can you provide some hints as to what the proper structure is? I've never had success uploading a DeepL glossary in XLS or CSV format and would appreciate some guidance.

Dear John,
The following steps are necessary for the successful creation and upload of a glossary for the DeepL plugin:

1. latest versions of the software: DeepL plugin 6.1.2.18 and Trados Studio 2022 SR2
2. a list of the term pairs in table format
3. copy the list into Word as text only
4. obtain a list of term pairs separated by the tab character
5. save this list in plain text format with UTF-8 encoding
6. make sure that no superfluous return character appears at the end of the list
7. change the name of the generated .txt file to .tsv (tab separated value)
8. upload this TSV file to the glossaries using the DeepL plugin (Manage glossaries).
9. Voilá !

 Karl Paul Berger John Fry 

Nice... although I think you can simplify this a little.  I would not use Word at all as most people have a decent text editor which is faster and more suitable for this kind of work.

1. latest versions of the software: DeepL plugin 6.1.2.18 and Trados Studio 2022 SR2
2. obtain (or create) a list of term pairs separated by the tab, or comma, character
3. save this list in plain text format with UTF-8 encoding
4. make sure that no superfluous return character appears at the end of the list
5. save the file with either the .tsv (tab separated value) or .csv (comma separated value) extension
6. upload this file to the glossaries using the DeepL plugin (Manage glossaries)
7. Voilá !

I think your detail is excellent for someone taking a glossary from a Word table, and this may well be a common format for many users, but if you were converting from multiterm or any other software then going straight to .tsv or .csv may be more appropriate.

You can also find this information here:

https://www.deepl.com/docs-api/glossaries

Search for "Supported Glossary Formats" as it's towards the end of the page and you'll get right to it.

Hi John and Paul,

Thanks very much for that information about importing glossaries into the DeepL plugin. I have around 600 entries and they import fine now, but I can't get at the Apply button to confirm that I want the entries in there. If I press Alt + Space and move the window, I can make the Apply button visible, but I still can't get there. Any ideas much appreciated. Thanks, Rowan

IMPORTANT PRECISION (for accented and special characters):

At the suggested step 3 (or step 2 in the updated version below), instead of UTF-8 encoding, I highly recommend using UTF-8 with BOM, otherwise all accented and special characters (including the curved apostrophe, used even in English) will be corrupted, as illustrated below:

So, to recapitulate, here is an updated version of what Paul said:

1. In the latest version of the software (DeepL plugin 6.1.2.18 and Trados Studio 2022 SR2), obtain (or create) a list of term pairs separated by the tab, or comma, character.
2. Save this list in plain text format using “UTF-8 with BOM” encoding.
3. Make sure that no superfluous return character appears at the end of the list.
4. Save the file with either the .TSV (tab separated value) or .CSV (comma separated value) extension.
5. Upload this file to the glossaries using the DeepL plugin (Manage glossaries).
   
   

ALTERNATIVE STEPS IF IMPORTING A GLOSSARY CREATED WITH DEEPL IN THE FIRST PLACE
Also, one thing to know is that if you first create your glossary with DeepL's web interface, terms will probably include superfluous straight quote brackets. So, in order to convert the .CSV file that will be exported from DeepL web, here's what I suggest:

1. Right click on the .CSV file and select either “Edit in Notepad” or “Open with” > “Notepad”.
2. Search and replace the " character by nothing.
3. Make sure that no superfluous return character appears at the end of the list.
4. Save the file in plain text format using “UTF-8 with BOM” encoding; once saved, it will normally retain its .CSV extension.
5. Close the file in Notepad, and upload it to the glossaries using the DeepL plugin (Manage glossaries).

IMPORTANT PRECISION (for accented and special characters):

At the suggested step 3 (or step 2 in the updated version below), instead of UTF-8 encoding, I highly recommend using UTF-8 with BOM, otherwise all accented and special characters (including the curved apostrophe, used even in English) will be corrupted, as illustrated below:

So, to recapitulate, here is an updated version of what Paul said:

1. In the latest version of the software (DeepL plugin 6.1.2.18 and Trados Studio 2022 SR2), obtain (or create) a list of term pairs separated by the tab, or comma, character.
2. Save this list in plain text format using “UTF-8 with BOM” encoding.
3. Make sure that no superfluous return character appears at the end of the list.
4. Save the file with either the .TSV (tab separated value) or .CSV (comma separated value) extension.
5. Upload this file to the glossaries using the DeepL plugin (Manage glossaries).
   
   

ALTERNATIVE STEPS IF IMPORTING A GLOSSARY CREATED WITH DEEPL IN THE FIRST PLACE
Also, one thing to know is that if you first create your glossary with DeepL's web interface, terms will probably include superfluous straight quote brackets. So, in order to convert the .CSV file that will be exported from DeepL web, here's what I suggest:

1. Right click on the .CSV file and select either “Edit in Notepad” or “Open with” > “Notepad”.
2. Search and replace the " character by nothing.
3. Make sure that no superfluous return character appears at the end of the list.
4. Save the file in plain text format using “UTF-8 with BOM” encoding; once saved, it will normally retain its .CSV extension.
5. Close the file in Notepad, and upload it to the glossaries using the DeepL plugin (Manage glossaries).

Louis LaBonté 

I like the belt and braces approach... although interestingly I don't need to add the BOM in my text editor.  I just save it as UTF-8 and that is enough.  Definitely doesn't hurt though so thank you for this improved suggestion.