Error when removing aliases with QuickStatements in background mode

**Steps to replicate the issue**:
I want to remove a set of aliases from different items in different languages using QuickStatements, so I:

1. go to https://quickstatements.toolforge.org/#/batch
2. insert the following commands:
```
-Q745 Ahy "մորի"
-Q17592 Aast "Avestruz"
-Q7045 Aast "Samartín"
-Q11575 Azh "玉米"
```
3. add a summary
4. click on "Import V1 commands"
5. click on "Run in background"

**What happens?**:
An example batch is [here](https://quickstatements.toolforge.org/#/batch/108337).
As you can see, all the commands produce an error.

**What should have happened instead?**:
All the aliases should have been removed.

**Other information**
If I try to click on "Run" instead of ("Run in background"), then everything is fine.

Note that it is not a Unicode-related problem, since it occurs even when only the second row is specified (see e.g. [this batch](https://quickstatements.toolforge.org/#/batch/108338)).

Start with the batch entry point at https://quickstatements.toolforge.org/#/batch and compare the normal Run path with Run in background using batches 108337 and 108338. Reproduce the alias-removal commands, then trace the background execution path. Done means the commands remove all listed aliases without errors, including the non-Unicode case.

Written by the indexing model from the issue text.