Wordpress ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\Psr7\uri_for()

Hi,

I have been using your tool for ages. And some weeks ago my backup job fails every time with the following error:
ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\Psr7\uri_for()

Accordingly, I have no backup now. :-(

Steps to reproduce the behavior:

1. Click Run job now
2. Wait some minutes then you can see it fails
3. No backup

Expected behavior
Run without error and do backup

System (please complete the following information):

• OS: Azure Linux Webapp
• Wordpress version: 5.7.1
• BackWPUp Version: 3.8.0

Detailed error log:

[INFO] BackWPup 3.8.0; A project of Inpsyde GmbH
[INFO] WordPress 5.7.1
[INFO] Log Level: Normal
[INFO] BackWPup job: Weekly Backup
[INFO] Logfile is: backwpup_log_daf44a_2021-04-27_20-37-37.html
[INFO] Backup file is: 2021-04-27_20-37-37_6FZH2WCQ01.tar.gz
[27-Apr-2021 20:37:38] 1. Try to backup database …
[27-Apr-2021 20:37:39] Connected to database on myprod.mysql.database.azure.com
[27-Apr-2021 20:37:41] Added database dump "backup.sql.gz" with 1,75 MB to backup file list
[27-Apr-2021 20:37:41] Database backup done!
[27-Apr-2021 20:37:41] 1. Trying to make a list of folders to back up …
[27-Apr-2021 20:39:14] Added "wp-config.php" to backup file list
[27-Apr-2021 20:39:14] 3309 folders to backup.
[27-Apr-2021 20:39:14] 1. Trying to generate a file with installed plugin names …
[27-Apr-2021 20:39:15] Added plugin list file "pluginlist.2021-04-27.txt.gz" with 2,13 KB to backup file list.
[27-Apr-2021 20:39:15] 1. Trying to generate a manifest file …
[27-Apr-2021 20:39:15] Added manifest.json file with 5,36 KB to backup file list.
[27-Apr-2021 20:39:32] 1. Trying to create backup archive …
[27-Apr-2021 20:39:32] Compressing files as TarGz. Please be patient, this may take a moment.
[27-Apr-2021 20:45:38] Backup archive created.
[27-Apr-2021 20:45:38] Archive size is 87,03 MB.
[27-Apr-2021 20:45:38] 19577 Files with 264,92 MB in Archive.
[27-Apr-2021 20:45:38] 1. Trying to send backup file to S3 Service …
[27-Apr-2021 20:45:40] ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\Psr7\uri_for()
[27-Apr-2021 20:45:54] 2. Trying to send backup file to S3 Service …
[27-Apr-2021 20:45:54] ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\Psr7\uri_for()
[27-Apr-2021 20:46:06] 3. Trying to send backup file to S3 Service …
[27-Apr-2021 20:46:06] ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\Psr7\uri_for()
[27-Apr-2021 20:46:08] ERROR: Step aborted: too many attempts!
[27-Apr-2021 20:46:08] 1. Try sending backup to a Microsoft Azure (Blob) …
[27-Apr-2021 20:46:09] ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\choose_handler()
[27-Apr-2021 20:46:17] 2. Try sending backup to a Microsoft Azure (Blob) …
[27-Apr-2021 20:46:17] ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\choose_handler()
[27-Apr-2021 20:46:21] 3. Try sending backup to a Microsoft Azure (Blob) …
[27-Apr-2021 20:46:21] ERROR: Exception caught in Error: Call to undefined function GuzzleHttp\choose_handler()
[27-Apr-2021 20:46:28] ERROR: Step aborted: too many attempts!
[27-Apr-2021 20:46:28] 1. Trying to check database …
[27-Apr-2021 20:46:30] Database check done!
[27-Apr-2021 20:46:30] ERROR: Job has ended with errors in 533 seconds. You must resolve the errors for correct execution.

No repository files or tests are named. Start by reproducing the failure through Run job now and trace the S3 and Microsoft Azure Blob upload paths associated with the logged missing functions; done means the backup completes and both upload attempts no longer error.